Abstract:
A system and method are disclosed to prevent a reduction in the number of I/O devices which can be connected when building a PCIe topology by connecting I/O devices to a computer via a PCIe switch. A switch with which a computer and I/O devices are connected includes: a first PCI-PCI bridge which is positioned on the computer side; a second PCI-PCI bridge which is positioned on the I/O device side; trapper units which trap packet data which is inputted into the switch; a packet routing unit which transfers packet data to the I/O devices; and a management processor which is connected to the trapper units and provides the computer a virtual PCI-PCI bridge and a virtual link by execution of a program. The trapper units adjudicate the destination of the packet data which is transferred from the computer.

Description:
TECHNICAL FIELD 
     The present invention relates to a computer system and a switch and packet transfer control method used therein, especially relates to transfer control over packet data by PCIe switches in a computer system in which plural computers and plural I/O devices are connected using the PCIe switches. 
     BACKGROUND ART 
     PCI Express (hereinafter called PCIe) is one type of an extended bus used in a computer and prescribed by PCI Special Interest Group (PCI-SIG). PCIe adopts a serial transfer interface and full duplex. Data transfer according to the PCIe is performed substantially like the transmission and reception of packet data (hereinafter merely called a packet) in a network and a transmission line of a packet is called PCIe link. 
     The PCIe includes Root complex, Endpoint and a PCIe switch as a component. The Root complex is a function for linking a CPU and a PCIe link. The Root complex is generally built in an I/O controller in a computer. The Endpoint is a function at an end of the PCIe link. The Endpoint is generally built in an I/O device. 
     The PCIe switch has a function for increasing the number of PCIe links and relaying a packet and is configured by plural PCI-PCI bridges. The PCI-PCI bridge has a function for determining whether a received packet is to be passed or not. The PCIe switch and the I/O device are connected via the PCIe link by Root Complex in the computer. The topology of PCIe components connected to Root Complex is called PCIe topology. 
     PCI Manager (hereinafter called PCIM) performs the management and the control of PCIe topology such as the generation, the deletion and a change of the PCIe topology. Since PCIM is installed as software, it can also be executed in a computer; however, the PCIM is generally installed in a supervisor processor (SVP) from a viewpoint of security. The PCIM gives a number called a bus number to respective PCIe links which the PCIM recognizes. In the PCIe, each PCIe link is identified using its bus number. 
     Further, the PCIM reads a number called a device number given beforehand from each PCI-PCI bridge and each I/O device which the PCIM respectively recognizes. In the PCIe, each PCI-PCI bridge and each I/O device are identified using their device numbers. For example, when a packet is transmitted to a certain I/O device, the packet can be correctly transmitted to a device which is a destination of the packet by adding the information of a bus number of a PCIe link connected to the I/O device and a device number of the I/O device to the destination of the packet. 
     As for routing control between an I/O device and a server using a data switch, technique that the data switch is connected to a proxy controller, a packet is classified into a packet for data transfer and a packet for control and the packet for control is processed in the proxy controller is disclosed in Patent Literature 1 for example. Further, access from at least one server to a virtual I/O device is described. 
     In addition, in Patent Literature 2 for example, technique that a module acquired by integrating a sorter of a packet and a built-in processor respectively called a configuration entity is built in a PCIe switch, a packet is classified into a packet for data transfer and a packet for control and the packet for control is processed in the built-in processor is disclosed. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Publication of United States Patent No. 2009/0150563 
         Patent Literature 2: U.S. Pat. No. 7,752,376 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The PCIM recognizes a PCIe link connected to an I/O controller, a PCIe switch and an I/O device based upon PCIe topology. Therefore, correlation between the PCIe link and a bus number is uniquely determined in the PCIe topology. 
     For example, when multistage PCIe switches are connected to an I/O controller and an I/O device is connected to the PCIe switch at an end, multiple PCIe links each of which connects PCI-PCI bridges are required. Although the I/O device is not connected to the PCIe link, a bus number is given to the PCIe link by PCIM. In the meantime, in the specification of PCIe, the number of available bus numbers is limited to 256. Therefore, the number of bus numbers which can be given to the PCIe links for connecting the I/O device decreases. Accordingly, a problem occurs that the number of I/O devices which PCIM can recognize decreases. In Patent Literatures 1 and 2, no special measure to settle the above-mentioned problem is disclosed. 
     A first object of the present invention is to prevent the number of connectable I/O devices from decreasing when a computer and the I/O device are connected via a PCIe switch and PCIe topology is configured. 
     Further, in the PCIe switch using the technique disclosed in Patent Literature 2, the built-in processor processes input all packets for control. Therefore, a load is applied to the built-in processor and capability for processing the packets may be deteriorated. 
     A second object of the present invention is to reduce a load of a processor included in the PCIe switch and to prevent capability for processing packets from being deteriorated. 
     Solution to Problem 
     It is desirable that a computer system according to the present invention is configured as a computer system based upon a computer system where packet data is transferred via switches connected to a computer and an I/O device and having a characteristic that the switch is provided with a first PCI-PCI bridge arranged on the side of the computer, a second PCI-PCI bridge arranged on the side of the I/O device, a trapper unit that traps packet data input to the switch and a packet routing unit that transfers the packet data to the I/O device, the switch is further provided with a management processor which is connected to the trapper unit and which provides a virtual PCI-PCI bridge and a virtual link to the computer by the execution of a program, the trapper unit determines a destination of the packet data transferred from the computer, when the destination is the I/O device as a result of the determination, the trapper unit passes the packet data without trapping it and the packet data is transferred to the I/O device via the packet routing unit and the second PCI-PCI bridge, when the destination is the virtual PCI-PCI bridge as the result of the determination, the trapper unit traps the packet data and transmits it to the management processor, and the management processor transmits packet data for a response to the computer via the first PCI-PCI bridge according to the packet data. 
     According to the desirable example, the switch is a semiconductor switch provided with the management processor and a memory in the computer system, and the memory holds address space of the virtual PCI-PCI bridge. 
     Further, it is desirable that in the computer system, the operation of the management processor is executed by a processor with which the computer is provided. 
     In addition, it is desirable that in the computer system, the trapper unit is provided with a PCIe packet receiver that receives packet data transmitted from the computer, a comparing unit that stores a range of addresses which the PCI-PCI bridge uses, a bus number and a device number, compares them with a destination of the input packet data and judges whether the packet data is to be trapped or not, a buffer that temporarily stores the packet data transferred to the management processor, an input-output unit that transfers the packet data to the management processor, and a PCIe packet transmitter that transmits the packet data whose destination transferred from the management processor is an I/O controller. 
     Further, it is desirable that in the computer system, the computer can recognize the virtual PCIe topology provided by the management processor by a program operated by itself, the virtual PCIe topology includes a virtual PCI-PCI bridge as the PCI-PCI bridges that connect PCIe switches and a virtual PCIe link that connects the computer and the PCIe switch and connects the PCIe switch and the I/O device, the PCI-PCI bridge included in the physical configuration of the computer system is distinguished as a physical PCI-PCI bridge, address space of such a PCI-PCI bridge that its virtual PCI-PCI bridge and its physical PCI-PCI bridge can be correlated by one to one is realized in that of the physical PCI-PCI bridge, address space of a virtual PCI-PCI bridge that cannot be correlated with the corresponding physical PCI-PCI bridge by one to one is secured in the memory, and the management processor accesses the address space in the memory of the virtual PCI-PCI bridge, acquires a response from the memory if necessary, and transmits the response to the trapper unit. 
     Furthermore, it is desirable that in the computer system, a bus number given to the I/O device is changed by changing the correlation between the virtual PCI-PCI bridge and the physical PCI-PCI bridge. 
     It is desirable that the switch according to the present invention is configured as a PCIe switch based upon a PCIe switch which is connected to a computer and an I/O device and which transfers packet data and having a characteristic that the switch is provided with a first PCI-PCI bridge arranged on the side of the computer and a second PCI-PCI bridge arranged on the side of the I/O device, a trapper unit that traps packet data input to the switch, a packet routing unit that transfers the packet data to the I/O device, and a management processor which is connected to the trapper unit and which provides a virtual PCI-PCI bridge and a virtual link to the computer by the execution of a program, the trapper unit determines a destination of the packet data transferred from the computer, when the destination is the I/O device as a result of the determination, the trapper unit passes the packet data without trapping it and the packet data is transferred to the I/O device via the packet routing unit and the second PCI-PCI bridge, when the destination is the virtual PCI-PCI bridge as the result of the determination, the trapper unit traps the packet data and transmits it to the management processor, and the management processor transmits packet data for a response to the computer via the first PCI-PCI bridge according to the packet data. 
     Further, it is desirable that the switch is provided with a memory that holds address space of the virtual PCI-PCI bridge, and the management processor accesses the address space of the virtual PCI-PCI bridge in the memory, acquires a response from the memory if necessary, and transmits the response to the trapper unit. 
     In addition, it is desirable that in the PCIe switch, the trapper unit is provided with a PCIe packet receiver that receives packet data transmitted from the computer, a comparing unit that stores a range of addresses which the PCI-PCI bridge uses, a bus number and a device number, compares them with a destination of input packet data and judges whether the packet data is to be trapped or not, a buffer that temporarily stores packet data transferred between the buffer and the management processor, an input-output unit that transfers packet data from/to the management processor, and a PCIe packet transmitter that transmits packet data whose destination transferred from the management processor is an I/O controller. 
     Further, it is desirable that in the PCIe switch, virtual PCIe topology includes a virtual PCI-PCI bridge as a PCI-PCI bridge that connects PCIe switches and a virtual PCIe link that connects the computer and the PCIe switch and connects the PCIe switch and the I/O device, the PCI-PCI bridge included in the physical configuration of the computer system is distinguished as a physical PCI-PCI bridge, address space of such a PCI-PCI bridge that its virtual PCI-PCI bridge and its physical PCI-PCI bridge can be correlated by one to one is realized in that of the physical PCI-PCI bridge, and address space of a virtual PCI-PCI bridge that cannot be correlated with the corresponding physical PCI-PCI bridge by one to one is secured in the memory. 
     It is desirable that a packet transfer control method according to the present invention is based upon a packet transfer control method for controlling the transfer of packet data using PCIe switches connected to a computer and an I/O device and has a characteristic that a trapper module in a PCIe switch determines whether packet data is to be trapped or not, referring to a destination of the packet data input to the PCIe switch, in the case of packet data whose destination is a PCI-PCI bridge as a result of the determination, the trapper module traps the packet data and transfers it to a management processor, in the case of packet data whose destination is the I/O device as the result of the determination, the trapper module transfers the packet data to the I/O device without trapping it, the management processor accesses address space of the PCI-PCI bridge realized in a memory connected to the management processor when the management processor receives the packet data transferred from the trapper module, acquires a response from the memory if necessary, and transmits the response to the trapper module and the trapper module transmits the response acquired from the management processor to the computer. 
     Further, it is desirable that a packet transfer control method according to the present invention is based upon a packet transfer control method for controlling packet data using PCIe switches connected to a computer and an I/O device, and has a characteristic that the packet transfer control method is provided with preparing beforehand a virtual PCI-PCI bridge and virtual links in a memory connected to a management processor that manages PCI-PCI bridges, storing beforehand virtual PCI-PCI bridges and virtual links in the memory connected to the management processor that manages the PCI-PCI bridges, receiving packet data transferred from the computer via the first PCI-PCI bridge arranged on the side of the computer by the PCIe switch, determining a destination of the received packet data by the trapper unit of the PCIe switch, passing the packet data without trapping it by the trapper unit when the destination is the I/O device as a result of the determination and transferring the packet data to the I/O device via the second PCI-PCI bridge arranged on the side of the I/O device, trapping the packet data by the trapper unit when the destination is the virtual PCI-PCI bridge as the result of the determination and transmitting the packet data to the management processor connected to the trapper unit, and preparing packet data for a response referring to the memory by the management processor according to the packet data received from the trapper unit and transmitting the packet data for the response to the computer via the first PCI-PCI bridge. 
     Advantageous Effects of Invention 
     According to the present invention, when the multistage PCIe switches are connected to the computer, the I/O device is connected to the PCIe switch at the end and the PCIe topology is configured, the number of connectable I/O devices can be prevented from decreasing. Especially, access to virtual PCIe topology can be realized in PCIe topology having different physical configurations, and the virtual PCIe topology that does not depend upon physical connection can be realized. Hereby, PCIM can flexibly allocate a bus number to the PCIe link to which the I/O device is connected. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  The configuration of a computer system in one embodiment. 
         FIG. 2  The configuration of virtual PCIe topology in one embodiment. 
         FIG. 3  The configuration of a trapper module in one embodiment. 
         FIG. 4  A packet according to PCIe in one embodiment. 
         FIG. 5  A flowchart showing the operation of the trapper module in one embodiment. 
         FIG. 6  A block diagram showing address space in a memory in one embodiment. 
         FIG. 7  Correlation between a bus number and a virtual PCIe link in one embodiment. 
         FIG. 8  A flowchart showing a process of firmware operated in a management processor in one embodiment. 
         FIG. 9  A flowchart showing a process when access from an I/O controller to a virtual PCI-PCI bridge occurs in one embodiment. 
         FIG. 10  A flowchart showing the process when the access from the I/O controller to the virtual PCI-PCI bridge occurs in one embodiment. 
         FIG. 11  A flow of a packet when access from the I/O controller to the virtual PCI-PCI bridge occurs in one embodiment. 
         FIG. 12  A flow of a packet when the access from the I/O controller to the virtual PCI-PCI bridge occurs in one embodiment. 
         FIG. 13  A flow of a packet when the access from the I/O controller to the virtual PCI-PCI bridge occurs in one embodiment. 
         FIG. 14  A flow of a packet when access from the I/O controller to an I/O device occurs in one embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, an embodiment of the present invention will be described below. 
       FIG. 1  shows the configuration of a computer system in one embodiment. 
     This computer system includes plural computers  100 - 1  to  100 - 4  and plural I/O devices  120 - 1  to  120 - 4 , and these are mutually connected via plural PCIe switches  110 - 1  to  110 - 4 . As the PCIe switch is configured by a semiconductor integrated circuit (LSI), it may be called a PCIe switch LSI. (Plural computers, the plural I/O devices and the plural PCIe switches are merely shown as  100 ,  120 ,  110  unless they are especially distinguished. Reference numerals of the other components are also similar.) The computer  100  is provided with a processor (CPU)  102  that executes various data processing by the execution of a program, a memory  103  and an I/O controller  101  connected to the PCIe switch. 
     The PCIe switch  110  is configured by PCI-PCI bridges  111  and  112  or  112  and  113 , trapper modules  114 , a packet routing unit  115 , a management processor  116  and a memory  117 . 
     The trapper module  114  and the management processor  116  are connected via a particular bus  118  that does not comply with the specification of PCIe. The computer  100  and the PCIe switch  110  are connected via a PCIe link  130 . The different PCIe switches  110  are connected via a PCIe link  140 . The PCIe switch  110  and the I/O device  120  are connected via a PCIe link  150 . 
     Referring to  FIG. 3 , the configuration of the trapper module  114  will be described below. 
     The trapper module  114  has a configuration which is characteristic of the present invention and is provided with a processing function for judging whether a packet from the PCI-PCI bridge is to be trapped or not according to a destination of the packet. As shown in  FIG. 3 , the trapper module  114  is configured by a PCIe packet receiver  301 , a packet destination comparing unit  302 , a buffer  303 , an input-output unit  304  from/to the management processor and a PCIe packet transmitter  305 . 
     The PCIe packet receiver  301  has a function for receiving a packet from the I/O controller  101 . The packet destination comparing unit  302  determines whether a destination of the received packet is the PCI-PCI bridge or not. As shown in  FIG. 4 , a destination  401  of a packet is included in a header of the PCIe packet  400 . The packet destination comparing unit  302  judges whether the packet is to be trapped or not by referring to the destination  401  of the packet. 
     The destination  401  of the packet has two types of formats. The first type is the format in which a destination is specified by an address. The second type is the format in which a destination is specified by a bus number and a device number. The packet destination comparing unit  302  stores a range of addresses which the PCI-PCI bridge uses, bus numbers and device numbers, compares with the destination  401  of the input packet, and judges whether the packet is to be trapped or not. The range of addresses which the PCI-PCI bridge uses, the bus numbers and the device numbers are transferred to the trapper module  114  via the particular bus  118  from the management processor  116 . 
     The trapper module  114  does not distinguish between a packet for data transfer and a packet for control and judges whether a packet is to be trapped or not depending upon a destination of the packet. Therefore, if a destination of a packet is the I/O device when the packet for control is received, the trapper module passes the packet without transferring the packet to the management processor. Therefore, the present invention can reduce a load of the management processor, compared with a case that all packets for control are transferred to the management processor. In addition, capability for processing a packet can be prevented from being deteriorated. 
     The buffer  303  temporarily stores a packet so as to transfer the packet between the trapper module  114  and the management processor  116  described later. The input-output unit  304  from/to the management processor is connected to the management processor  116  and transfers a packet via the particular bus. The PCIe packet transmitter  305  transmits a packet for a response whose destination transferred from the management processor is the I/O controller  101 . 
     The trapper module  114  can trap a packet whose destination is a PCI-PCI bridge that does not physically exist because the trapper module exists between PCI-PCI bridges  111 - 1  to  111 - 4  which are connected to the I/O controller  101  and the packet routing unit  115 - 1  or  115 - 2  as shown in  FIG. 1 . 
     In this embodiment, a routing control mechanism of a packet in the computer system is configured by combining plural pieces of the same PCIe switches  110 . Each function of the trapper module  114  is made effective when the PCIe switch  110  is connected to the I/O controller  101 . Accordingly, in the trapper modules  114 - 1  to  114 - 4  included in the PCIe switches  110 - 1 ,  110 - 2 , their functions effectively act and routing control using the management processor is performed. 
     In the meantime, in the trapper modules  114 - 5  to  114 - 8  in the PCIe switches  110 - 3 ,  110 - 4  which are not connected to the I/O controller  101 , the function of the trapper module  114  is nullified and all packets are passed. That is, since no function of the trapper module  114  acts (is necessary), routing control without the aid of the management processor is applied to all packets. 
     The PCIe link performs communication in full duplex in which a signal line for transmission and a signal line for reception are separated. Therefore, each signal line connected to the trapper modules  114 - 1  to  114 - 4  is classified into a signal line connected to the PCI-PCI bridge  111  and a signal line connected to the packet routing unit  115 . The trapper module  114  applies the function of the trapper module  114  to a signal input from the PCI-PCI bridge  111 . Therefore, each function of the trapper module  114  is applied to a packet input from the PCI-PCI bridge  111 . Each function of the trapper module  114  is not applied to a signal input from the packet routing unit  115 . Therefore, a packet input from the packet routing unit  115  passes the PCI-PCI bridge  111 . 
     In the present invention, the trapper module  114  and the management processor  116  are not integrated and are different modules. Therefore, the concentrated input of packets to one trapper module  114  is avoided by arranging the plural trapper modules  114  and the packets can be transferred to the management processor  116  at high speed. 
       FIG. 5  is a flowchart showing the operation of the trapper module  114 . 
     This process is executed when the trapper module  114  receives a packet from the I/O controller  101  (S 501 ). It is determined whether a destination  401  of the received packet is the PCI-PCI bridge or not (S 502 ). In the case of Yes (that is, in a case that the destination is the PCI-PCI bridge) as a result of the determination, processing proceeds to S 503  and in the case of No (in a case that the destination is not the PCI-PCI bridge), the processing proceeds to S 504 . 
     In the step S 503 , the trapper module  114  traps the received packet and transfers it to the management processor  116 . Processing in a case that the packet is transferred to the management processor  116  will be described later referring to  FIG. 8 . 
     Further, in the step S 504 , the trapper module  114  passes the received packet without changing the destination. In this case, the packet is transferred to an address included in the destination  401  or to any I/O device  120  specified by a bus number and a device number. 
     When either processing in S 503  or S 504  is completed, the trapper module  114  finishes the operation. 
     Referring to  FIG. 1  again, the packet routing unit  115  is connected to the trapper module  114 , the management processor  116  and the PCI-PCI bridge  112  or  113  and has a function for transferring the input packet to its destination. The packet routing unit is not required to depend upon the specification of PCIe and if only the packet reaches the destination, a type of routing does not come into question. 
     The management processor  116  is a processor that operates firmware which manages the PCI-PCI bridge. The operation of the firmware will be described later. Further, the memory  117  is connected to the management processor  116 . The functions of the management processor  116  and the memory  117  are validated when the PCIe switch  110  is connected to the I/O controller  101  like the trapper module  114 . Corresponding to it, the functions of the management processors  116 - 1 ,  116 - 2  are validated and in the meantime, the functions of the management processors  116 - 3 ,  116 - 4  are nullified. 
     Also for the memory  117 , the functions of the memories  117 - 1 ,  117 - 2  are validated and the functions of the memories  117 - 3 ,  117 - 4  are nullified. 
     In this embodiment, the management processor  116  and the memory  117  are mounted in the PCIe switch  110 ; however, the present invention is not limited to this. For example, the management processor  116  or the memory  117  or both may also be arranged outside the PCIe switch  110 . 
     Further, in this embodiment, the management processor  116  is built in the PCIe switch  110 ; however, the present invention is not limited to this, and in place of the management processor  116 , the CPU  102  included in the computer  100  may also be used. Similarly, in place of the memory  117 , the memory  103  included in the computer  100  may also be used. 
       FIG. 2  shows PCIe topology which a program operated in the computer  100 - 1  for example recognizes. Programs operated in the computers  100 - 2  to  100 - 4  can also be similarly recognized. As PCIe topology is logical unlike a physical configuration, it is called virtual PCIe topology in this case. 
     The virtual PCIe topology is PCIe topology which the program operated in the computer  100  recognizes. The program operated in the computer  100  recognizes the virtual PCIe topology by searching the PCIe link, the PCI-PCI bridge and the I/O device. 
     PCI-PCI bridges  221 ,  222 ,  231 ,  232  included in the virtual PCIe topology shall be called a virtual PCI-PCI bridge below. Further, PCIe switches  220 ,  230  shall be called virtual PCIe switch. Further, PCIe links  250 ,  260 ,  270 ,  280 ,  290  shall be called virtual PCIe link. 
     The virtual PCIe switch  220  is connected to the computer  100 - 1  via the virtual PCIe link  250 . Further, plural virtual PCIe switches  230  are connected to the virtual PCIe switch  220  via the virtual PCIe link  270 . Further, the plural I/O devices  240  are connected to the virtual PCIe switches  230  via the virtual PCIe link  290 . 
     In  FIG. 2 , the PCIe switches  220 ,  230  are connected to the I/O controller  101 - 1  with two stages overlapped in each PCIe switch; however, the present invention is not limited to this. For example, the PCIe switch may also be configured by one stage or three or more stages. A user can arbitrarily determine the virtual PCIe topology based upon the number of the computer  100  and the I/O device  120 . Accordingly, it can be said that there is no dependence between the number of the PCIe switches shown in  FIG. 1  and the number of the virtual PCIe switches shown in  FIG. 2 . In the specification of PCIe, since the maximum number of available bus numbers connected to one computer is 256, the maximum number of the PCI-PCI bridges (that is, the PCIe switches  220 ) connected to the computer  100 - 1  is 8 and the number of the PCI-PCI bridges connected to the PCIe switch  220  is 32 when the example shown in  FIG. 2  is extended. 
     To distinguish from the virtual PCI-PCI bridge, the PCI-PCI bridge included in the physical configuration of the computer system shown in  FIG. 1  will be called the physical PCI-PCI bridge below. 
     Address space of the virtual PCI-PCI bridge can also be all realized in the memory  117 . However, when the address space of all the virtual PCI-PCI bridges is realized in the memory  117 , the capacity of the memory  117  increases. Further, in this case, since the management processor  116  performs all the processing of the virtual PCI-PCI bridges, a load of the management processor  116  increases. 
     For the above-mentioned reason, in this embodiment, it is desirable that when the virtual PCI-PCI bridge and the physical PCI-PCI bridge can be correlated by one to one, the correlation is utilized. Hereby, the address space of the correlatable PCI-PCI bridges is realized in an address of the physical PCI-PCI bridge. In the meantime, address space of the virtual PCI-PCI bridge which is not correlated is realized in the memory  117 . When virtual PCIe topology and the physical configuration of the computer system shown in  FIG. 1  are determined, the following correlation can be determined. 
     The virtual PCI-PCI bridges shown in  FIG. 2  can be classified into the following three types. The first type is the virtual PCI-PCI bridge  221  connected to an I/O controller  101 - 1 , the second type is the virtual PCI-PCI bridge  232  connected to the I/O device  240  in the virtual PCIe topology, and the third type is the virtual PCI-PCI bridges  222 ,  231  that connect the virtual PCIe switches  220 ,  230 . 
     The physical PCI-PCI bridges included in the computer system shown in  FIG. 1  can be classified into the following three types. The first type is the physical PCI-PCI bridge  111  connected to the I/O controller  101 , the second type is the physical PCI-PCI bridge  113  connected to the I/O device  120  in the physical configuration, and the third type is the physical PCI-PCI bridge  112  that connects the PCIe switches  110 - 1 ,  110 - 2  and the PCIe switches  110 - 3 ,  110 - 4 . 
     For example, when the computer  100 - 1  recognizes the I/O devices  120 - 1 ,  120 - 4  in the physical configuration, the virtual PCI-PCI bridge  221  and the physical PCI-PCI bridge  111 - 1  are correlated. Further, the virtual PCI-PCI bridge  232 - 1  and the physical PCI-PCI bridge  113 - 1  are correlated. Further, the virtual PCI-PCI bridge  232 - 2  and the physical PCI-PCI bridge  113 - 4  are correlated. In this case, the virtual PCI-PCI bridges  222 ,  231  are not correlated with the physical PCI-PCI bridge. Furthermore, as for the physical PCI-PCI bridge  112  that connects the different PCIe switches  110 , a function as the PCI-PCI bridge for judging whether a packet can be passed or not is nullified and all packets are passed. 
     When the virtual PCI-PCI bridge and the physical PCI-PCI bridge are correlated, the I/O devices are also correlated by one to one. In the case of the above-mentioned example, the I/O device  240 - 1  in the virtual PCIe topology and the I/O device  120 - 1  in the physical configuration are correlated. Further, the I/O device  240 - 2  in the virtual PCIe topology and the I/O device  120 - 4  in the physical configuration are correlated. 
     As described above, it is known that the address space of the virtual PCI-PCI bridge  221  is realized in the physical PCI-PCI bridge  111 . Further, the address space of the virtual PCI-PCI bridge  232  is realized in the physical PCI-PCI bridge  113 . The address space of the virtual PCI-PCI bridges  222 ,  231  is realized in the memory  117  as shown in  FIG. 6 . That is, using the example shown in  FIG. 2 , the address space of the virtual PCI-PCI bridges  222 - 1 ,  222 - 2 ,  222 - 3 ,  231 - 1 ,  231 - 2 ,  231 - 3  is secured in the memory  117 . 
     When PCIM recognizes the virtual PCIe topology shown in  FIG. 2 , the PCIM gives the virtual PCIe link a bus number as follows. 
     First, the device connected to the I/O controller  101  is searched. Then, the PCIM recognizes that the virtual PCI-PCI bridge  221  is connected to the I/O controller  101 . Hereby, the PCIM gives a bus number  0  to the PCIe link  250  that connects the I/O controller  101  and the virtual PCI-PCI bridge  221  (see  FIG. 7 ). 
     Next, the device connected to the virtual PCI-PCI bridge  221  is searched. Then, the PCIM recognizes that the virtual PCI-PCI bridges  222 - 1 ,  222 - 2 ,  222 - 3  are connected to the virtual PCI-PCI bridge  221 . Hereby, the PCIM gives a bus number  1  to the PCIe link  260  that connects the virtual PCI-PCI bridge  221  and the virtual PCI-PCI bridge  22 . 
     Next, the device connected to the virtual PCI-PCI bridge  222 - 1  is searched. Then, the PCIM recognizes that the virtual PCI-PCI bridge  231 - 1  is connected to the virtual PCI-PCI bridge  222 - 1 . Hereby, the PCIM gives a bus number  2  to the PCIe link  270 - 1  that connects the virtual PCI-PCI bridge  222 - 1  and the virtual PCI-PCI bridge  231 - 1 . 
     Correlation between the bus numbers and the virtual PCIe links when the PCIM continues the above-mentioned operation is shown in  FIG. 7 . Therefore, in the example in this embodiment, since the virtual PCIe link  290 - 1  is connected to the I/O device  120 - 1  in the physical configuration, a bus number  4  is given to the link. Further, since the virtual PCIe link  290 - 2  is connected to the I/O device  120 - 4  in the physical configuration, a bus number  5  is given to the link. 
     A bus number given to the I/O device changes by changing correlation between the virtual PCI-PCI bridge and the physical PCI-PCI bridge. For example, since the virtual PCIe link  290 - 4  is connected to the I/O device  120 - 4  in the physical configuration when the virtual PCI-PCI bridge  232 - 4  and the physical PCI-PCI bridge  113 - 4  are correlated, a bus number  9  is given to the link. As described above, the bus numbers can be flexibly allocated to the I/O devices. 
     Next, the processing of the firmware operated in the management processor  116  will be described, referring to  FIG. 8 . The processing of the firmware is executed when the management processor  116  receives a packet from the trapper module  114  (S 801 ) or receives a packet from the packet routing unit  115  (S 807 ). 
     First, a destination of a packet received from the trapper module  114  is recognized (S 802 ). As a result, it is determined whether or not the destination of the packet is the virtual PCI-PCI bridge correlated with the physical PCI-PCI bridge (S 803 ). When a result of the determination is right (Yes), processing proceeds to S 804  and when a result of the determination is false (No), the processing proceeds to S 805 . 
     In S 804 , a packet received from the trapper module  114  is transferred to the correlated physical PCI-PCI bridge. In the meantime, in S 805 , the firmware accesses the address space realized in the memory  117  of the virtual PCI-PCI bridge based upon a packet received from the trapper module. As a result of the processing in the firmware, when a response is required to be returned to the I/O controller, a response packet is generated and returned to the I/O controller  101  via the trapper module  114  (S 806 ). 
     Further, when a packet is received from the packet routing unit (S 807 ), the received packet is returned to the I/O controller  101  via the trapper module  114  (S 808 ). 
     The PCIe topology recognized by a program operated in the computer system shown in  FIG. 1  is the virtual PCIe topology shown in  FIG. 2 . Therefore, access from the I/O controller  101  to the virtual PCI-PCI bridges  221 ,  222 ,  231 ,  232  occurs. A flow of processing executed by the PCIe switch  110  when a packet of the access is input to the PCIe switch  110  shown in  FIG. 1  will be described referring to  FIGS. 9 and 10  below. 
     In this case, a case that the access to the virtual PCI-PCI bridges  221 ,  222 ,  231 ,  232  occurs means a case that the contents (for example, identification information of the PCI-PCI bridge) with which the PCI-PCI bridge is provided of the memory is read according to an instruction from the computer  100  or a case that a parameter of the PCI-PCI bridge is initialized (written) or a case that a parameter set in the PCI-PCI bridge is released for example. 
       FIG. 9  shows a flow of processing executed by the PCIe switch  110  when access from the I/O controller  101  to the virtual PCI-PCI bridges  222 ,  231  occurs in the virtual PCIe topology shown in  FIG. 2 . 
     When a packet is transmitted from the I/O controller  101  to the PCI-PCI bridge (S 901 ), the trapper module  114  traps the packet and sequentially writes it in the buffer  303  in the trapper module  114  (S 902 ). When the trapper module  114  finishes writing the trapped packet to the buffer  303 , the trapper module notifies the management processor  116  of trapping the packet (S 903 ). 
     When the management processor  116  receives the notice of trapping the packet, it transmits an instruction to read data in the buffer  303  to the trapper module  114  (S 904 ). Then, the trapper module  114  reads the data of the packet from the buffer  303  (S 905 ) and transmits the read data to the management processor  116  (S 906 ). 
     When the management processor  116  receives the data of the packet, it recognizes a destination of the packet ( 907 ). As a result, when the management processor recognizes that the destination of the packet is the virtual PCI-PCI bridges  222 ,  231  (S 907 ), the management processor  116  accesses address space for the virtual PCI-PCI bridges in the memory  117  (S 908 ). 
     Generally, no response is made to access to the memory in writing processing and a response including read data is made in reading processing. Therefore, when the contents of a packet to the virtual PCI-PCI bridge are writing processing, no response is made from the memory  117  to the management processor  116 . When the contents of a packet to the virtual PCI-PCI bridge are reading processing, the management processor  116  receives a response from the memory  117  (S 909 ). 
     When access from the I/O controller  101  is reading from memory space and access to configuration space that respectively require a packet for a response, the management processor  116  generates the packet for the response to the I/O controller  101  (S 910 ). The management processor  116  transmits the generated packet for the response to the trapper module  114  (S 911 ). 
     The trapper module  114  sequentially writes the packet for the response received from the management processor  116  to the buffer  303  in the trapper module  114  (S 912 ). 
     When the management processor  116  transmits all packets for responses to the trapper module  114 , it transmits an instruction to transmit the packets for responses to the I/O controller  101  to the trapper module  114  (S 913 ). Then, the trapper module  114  reads the packets for responses from the buffer  303  (S 914 ) and transmits the packets for responses to the I/O controller  101  (S 915 ). 
     When access from the I/O controller  101  such as writing to memory space requires no packet for a response, the management processor  116  notifies the trapper module  114  of the termination of processing (S 916 ). 
     When the transmission of the packets for responses to the I/O controller  101  is completed or when the notice of the termination of processing from the management processor  116  is received, the trapper module  114  deletes all the packets in the buffer  303  (S 917 ). 
     As a result, the processing of the computer system for the access from the I/O controller  101  to the virtual PCI-PCI bridges  222 ,  231  is finished. 
       FIG. 10  shows a flow of processing executed by the PCIe switch  110  when access from the I/O controller  101  to the virtual PCI-PCI bridge  232  occurs in the virtual PCIe topology shown in  FIG. 2 . Operation from S 901  to S 906  is the same as the processing shown in  FIG. 9 . 
     When the management processor  116  recognizes that a destination of a packet is the virtual PCI-PCI bridge  232  (S 1001 ), it searches the correlated physical PCI-PCI bridge (S 1002 ). The management processor changes the destination of the packet to the searched physical PCI-PCI bridge  113  (S 1003 ) and transfers the packet to the physical PCI-PCI bridge  113  (S 1004 ). 
     When the access from the I/O controller  101  is reading from memory space and access to configuration space that respectively require a packet for a response, the management processor  116  receives the packet for the response from the physical PCI-PCI bridge  113  (S 1005 ). 
     Afterward, operation from S 910  to S 917  is the same as that in the process shown in  FIG. 9 . As a result, the processing of the computer system for the access from the I/O controller  101  to the virtual PCI-PCI bridge  232  is finished. 
     Next, a concrete example of routing control in transmitting a packet in the computer system shown in  FIG. 1  when access from the I/O controller  101  to the virtual PCI-PCI bridges  221 ,  222 ,  231 ,  232  or to the I/O device  120  in the physical configuration occurs in the virtual PCIe topology shown in  FIG. 2  will be described. 
       FIG. 11  shows a flow of a packet in the computer system when access from the I/O controller  101 - 1  to the virtual PCI-PCI bridge  221  occurs in the virtual PCIe topology shown in  FIG. 2 . 
     In this case, before the packet reaches the trapper module  114 - 1 , the packet reaches the correlated physical PCI-PCI bridge  111 - 1 . Therefore, direct access is made from the I/O controller  101 - 1  to the physical PCI-PCI bridge  111 - 1  correlated with the virtual PCI-PCI bridge  221 . 
       FIG. 12  shows a flow of a packet in the computer system when access from the I/O controller  101 - 1  to the virtual PCI-PCI bridges  222 ,  231  occurs in the virtual PCIe topology shown in  FIG. 2 . 
     In this case, the trapper module  114 - 1  which is the closest to the I/O controller  101 - 1  traps the packet and transfers the packet to the management processor  116 - 1  using a dedicated bus. When the management processor  116 - 1  receives the packet, it performs the process shown in  FIG. 9  using firmware. The management processor  116 - 1  generates a packet for a response to the I/O controller  101 - 1  and returns the packet for the response via the trapper module  114 - 1 . 
       FIG. 13  shows a flow of a packet in the computer system when access from the I/O controller  101 - 1  to the virtual PCI-PCI bridge  232 - 1  occurs in the virtual PCIe topology shown in  FIG. 2 . 
     In this case, the trapper module  114 - 1  which is the closest to the I/O controller  101 - 1  traps the packet and transfers the packet to the management processor  116 - 1  using the particular bus  118 - 1 . When the management processor  116 - 1  receives the packet, it transfers the received packet to the physical PCI-PCI bridge  113 - 1  correlated with the virtual PCI-PCI bridge  232 - 1  with a destination of the packet changed as shown in  FIG. 10 . When the management processor  116 - 1  receives a response packet from the physical PCI-PCI bridge  113 - 1 , the management processor  116 - 1  returns the response packet to the I/O controller  101 - 1  via the trapper module  114 - 1 . 
       FIG. 14  shows a flow of a packet in the computer system when access from the I/O controller  101 - 1  to the I/O device  240 - 1  in the virtual PCIe topology occurs in the virtual PCIe topology shown in  FIG. 2 . In this case, the trapper module  114 - 1  traps no packet. Direct access is made from the I/O controller  101 - 1  to the I/O device  120 - 1  in the physical configuration correlated with the I/O device  240 - 1  in the virtual PCIe topology. 
     The preferred embodiment of the present invention has been described; however, the present invention is not limited to the embodiment, and further, the present invention may be variously modified. 
     For example, in the above-mentioned embodiment, each function of the trapper module  114  is validated when the PCIe switch  110  is connected to the I/O controller  101 , in the meantime, when the PCIe switch  110  is not connected to the I/O controller  101 , the functions of the trapper modules  114 - 5  to  114 - 8  in the PCIe switches  110 - 3 ,  110 - 4  are nullified, and all packets are passed. This is based upon a premise that all the PCIe switches  110  configured by a semiconductor integrated circuit have the same configuration including the trapper module. If a PCIe switch (a first PCIe switch) connected to the I/O controller  101  and a PCIe switch (a second PCIe switch) not connected to the I/O controller  101  can be configured by separate semiconductor integrated circuits, a function of the first PCIe switch provided with the trapper module can be ordinarily effectively utilized. In the meantime, since the function of the trapper module is nullified in the second PCIe switch, the second PCIe switch can be configured as the semiconductor integrated circuit provided with no trapper module at a first stage. 
     Further, in the embodiment, to inhibit an increase of the capacity of the memory  117  and an increase of the load of the management processor  116  in the address space of the virtual PCI-PCI bridge in the memory  117 , the virtual PCI-PCI bridge and the physical PCI-PCI bridge are possibly correlated by one to one. However, it is also possible to adopt another example that if the capacity of the memory  117  is sufficient and the load of the management processor  116  is also allowable, the address space of all the virtual PCI-PCI bridges is secured in the memory. When the memory has the address space of all the virtual PCI-PCI bridges, processing of whether the virtual PCI-PCI bridge and the physical PCI-PCI bridge are correlated by one to one (S 803  in  FIG. 8 ) is not required, and the management processor can process all received packets. 
     As described above, according to the preferred embodiment of the present invention, the virtual PCIe topology that does not depend upon physical connection can be realized. Therefore, the PCIM can flexibly allocate a bus number to the PCIe link connected to the I/O device. Even if the multistage PCIe switches are connected to the I/O controller and the I/O device is connected to the PCIe switch at the last stage, the problem that the number of the connectable I/O devices decreases can be solved. Further, the I/O controller can access all the virtual PCI-PCI bridges and all the I/O devices. 
     LIST OF REFERENCE SIGNS 
       100 : Computer  101 : I/O controller  102 : CPU  103 : Memory  110 : PCIe switch  111  to  113 : Physical PCI-PCI bridge  114 : Trapper module  115 : Packet routing unit  116 : Management processor  117 : Memory  118 : Particular bus  120 : I/O device  130  to  150 : PCIe link  220 : Virtual PCIe switch  221  to  222 : Virtual PCI-PCI bridge  230 : Virtual PCIe switch  231  to  232 : Virtual PCI-PCI bridge  240 : I/O device  250  to  290 : Virtual PCIe link  301 : PCIe packet receiver  302 : Packet destination comparing unit  303 : Buffer  304 : Input-output unit from/to management processor  305 : PCIe packet transmitter  400 : Packet  401 : Destination of packet