Abstract:
This computer system improves the ease with which a guest OS and remote device can be connected in a configuration combining pass-through technology and bus extension technology. The computer system is provided with a bus extending means for constructing a virtual bus to one or more remote devices connected via a network, and an executing means for configuring a virtual hardware environment and executing a host operating system for causing a guest operating system to control the remote device(s). The bus extending means comprises a communication controlling means for consulting a management table for managing relationships of correspondence between the guest operating system and the remote device(s) by using identification information that does not change depending on the startup state of the remote device(s), and controlling communication between the guest operating system and the remote device(s).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a computer system, a connection management method for a remote device, and a program recording medium, and particularly relates to a computer system, a connection management method for a remote device, and a program recording medium which perform input and output with a remote device via a network. 
       BACKGROUND ART 
       [0002]    A computer includes a CPU (Central Processing Unit) and a device which are connected each other via a system bus (hereinafter, referred to as ‘bus’ in some cases), and accesses the device according to an instruction of a program executed on the CPU. The access to the device is referred to as ‘input/output process’ or ‘I/O process’. Meanwhile, software which controls the device is referred to as an ‘OS (Operating System)’. Especially, software, which controls the device, out of OSs is referred to as a ‘device driver’. Moreover, a plurality of user programs are executed on the OS by control of the OS. In order that the user program may not access the device directly, there is a difference in execution authority between the OS and the user program. 
         [0003]    Xeon® or Atom of Intel Corporation are examples of the CPU. Linux®, Windows® of Microsoft Corporation or the like is an example of the OS. Moreover, PCI (Peripheral Component Interconnect), PCI-Express or the like is an example of the system bus. Furthermore, a hard disk drive (HDD), a network interface card (NIC), and the like are examples of the devices. 
         [0004]    The devices are scanned at a start-up time of the computer by BIOS (Basic Input/Output System), and are assigned IDs (Identifiers), memory areas and the like. At this time, the devices are not in an available state. After a start-up of the OS, by a device driver working for a device, to be operated by the OS, out of scanned devices, the device is initialized. By executing the initialization, the device enters into the available state. A set of ‘bus number/device number/function number (BDF) is an example of ID, and it is possible to uniquely specify the device, which is connected with the computer, by using the set. This set is a logical number, and may be changed in some cases according to the number, a configuration, or a scanning order of the devices. 
         [0005]    As a method for accessing the device for the running OS, two access methods are known. That is, one is the access method by issuing an I/O instruction and the other is the access method by MMIO (Memory-Mapped I/O). According to the former method, the I/O instruction is defined as an instruction set of the CPU. Therefore, the I/O instruction is outputted on the bus by the program&#39;s issuing the CPU instruction, and the access to the device is executed. Meanwhile, according to the latter method, when the OS accesses a specified memory address, the CPU or a chipset which controls the bus converts the memory address into an I/O process and outputs the I/O process on the bus. As a result, the access to the device is executed. As mentioned above, when BIOS scans the devices, since a size of memory space which the device requests is assigned in physical memory space, a memory access to the assigned physical memory space is converted into the I/O instruction. Hereinafter, the above-mentioned two methods for device access are collectively referred to as ‘I/O instruction’. When accessing the device, the memory address, ID or the like which identifies the device is designated by the I/O instruction. 
         [0006]    Recently, CPU&#39;s advance in performance permits some software overheads. Consequently, it is possible to make hardware virtualized, and to make a plurality of virtual hardware environments work on a single CPU. 
         [0007]      FIG. 9  is a block diagram exemplifying a configuration of a virtualized computer which makes such virtual hardware environments work. The OS which manages the virtual hardware environment shown in  FIG. 9  is referred to as a host OS X 110 . Meanwhile, the OS which is executed in the virtual hardware environment is referred to as a guest OS. While the number of the host OSs X 110  is one, it is possible to execute a plurality of the guest OSs (refer to signs X 101 -X 10   n ). Moreover, the guest OS may be different in a kind from the host OS X 110 , and the guest OS may be different in a kind from the other guest OSs. As an example of the host OS X 110  which provides the virtual hardware environment, kvm, VMWare® or the ESXi server of VMWare Inc., or the like is known. 
         [0008]    Generally, in the virtualized environment, instead of making the gust OS directly recognize a device B X 131  which is connected with the host OS X 110 , a virtualized device A (hereinafter, referred to as ‘virtual device’) is provided to the guest OS (for example, the guest OS X 101 ). That is, a device driver B, which the host OS X 110  has, controls the device B X 131 . Meanwhile, a device driver A, which the guest OS (for example, the guest OS X 101 ) has, controls the virtual device A. At this time, the host OS X 110  converts an I/O instruction, which the device driver A of the guest OS (for example, the guest OS X 101 ) issues, in such a way that a device driver of the device B X 131  can decode the I/O instruction. 
         [0009]    Meanwhile, PTL 1 discloses ‘pass-through technology’ managing the device by not the host OS but a device driver of the guest. According to the technology described by PTL 1, it is possible that the guest OS in place of the host OS can control some devices which are connected with the computer. 
         [0010]    Moreover, PTL 2 discloses a technology for extending the bus virtually. According to the technology described by PTL 2, it is possible to physically extend a distance between CPU and the device by transferring data, which flow on PCI-Express, through a network based on Ethernet® or the like. The PCI-Express bus transfers data in a form of packet. Here, the packet is a unit of a transfer for communication between two points, and generally a source address and a destination address are embedded in the packet. 
         [0011]      FIG. 10  is a block diagram showing a configuration of a computer which applies the bus extension technology described by PTL 2. Referring to  FIG. 10 , according to the technology described by PTL 2, an upstream bridge X 140  is arranged to connect to a CPU X 120 , and a downstream bridge X 420  is arranged to connect to a device (peripheral equipment X 130 ). Each of the bridges X 140  and X 420  encapsulates a packet, and de-capsulates the encapsulated packet (that is, releases encapsulation). By the above-mentioned encapsulation, it is possible to make data on the bus transferred through an Ethernet® network. Since also the Ethernet network transfers data in a form of packet, hereinafter, a packet which flows on a network is referred to as a ‘network packet’. 
         [0012]    PTL 3 discloses a method for facilitating connection of a USB (Universal Serial Bus) device in a virtualized environment. Specifically, PTL 3 discloses a virtual bus system having a device management unit which determines whether or not enumeration can be executed by referring to a permission list and a device management table for managing a connection state of another device. 
       CITATION LIST 
     Patent Literature 
       [0013]    [PTL 1] Japanese Patent Application Laid-Open Publication No. 2010-237737 
         [0014]    [PTL 2] Japanese Patent Application Laid-Open Publication No. 2007-219873 
         [0015]    [PTL 3] Japanese Patent Application Laid-Open Publication No. 2011-145827 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0016]    It is assumed that all disclosures of the above-mentioned patent documents are incorporated and described in the present DESCRIPTION by reference. The following analysis is achieved by the present inventor. By combining the pass-through technology ( FIG. 9 ) described by PTL 1, and the bus extension technology ( FIG. 10 ) described by PTL 2, it is possible to realize a computer system in which the guest OS directly controls the remote device connected through the network.  FIG. 11  shows an example of a plurality of remote apparatuses and a plurality of the guest OSs are directly connected by using the pass-through technology. Since the remote apparatuses may be arranged at locations different each other in some cases, a certain guest OS must be associated with the same remote apparatus every time. In the case of the example shown in  FIG. 11 , a guest OS-A controls a device A (bus number/device number/function number are  1 / 0 / 0  respectively), and a guest OS-B controls a device B (bus number/device number/function number are  2 / 0 / 0  respectively). 
         [0017]    However, according to the configuration which combines the pass-through technology and the bus extension technology, there is a problem that, in the case that some remote devices out of the plural remote devices are not started up, a method for designating the remote device, which is associated with the guest OS, is changed. For example, in the case of the example shown in  FIG. 11 , when a remote apparatus X 20 A is not started up, bus number/device number/function number of a device B existing in a remote apparatus B is changed from  2 / 0 / 0  to, for example,  1 / 0 / 0  respectively. Therefore, there is a possibility that the guest OS-A controls the device B. This is because a logical number is used for assigning the device number on the assumption that the identical device is included in the identical configuration (all of the devices are always started up) every time. Accordingly, a technique is proposed to provide a way in which the method for the guest OS&#39;s designating the remote device is not changed even if some remote devices out of the plural remote devices are not started up. 
         [0018]    An object of the present invention is to improve easiness of connection between the guest OS and the remote device in a configuration combining the pass-through technology and the bus extension technology. 
       Solution to Problem 
       [0019]    According to a first aspect, a computer system includes: a bus extending means which constructs a virtual bus to one or more remote devices connected via a network; and an executing means which configures a virtual hardware environment and executes a host operating system for causing a guest operating system to control the remote device, wherein the bus extending means comprises a communication controlling means which controls communication between the guest operating system and the remote devices by referring to a management table for managing correspondence between the guest operating system and the remote devices using identification information that does not change dependently on start-up states of the remote devices is provided. 
         [0020]    According to a second aspect, a connection management method for a remote device in a computer system including a bus extending means which constructs a virtual bus to one or more remote devices connected via a network and an executing means which configures a virtual hardware environment and executes a host operating system for causing a guest operating system to control the remote devices, the method includes: transferring a scan instruction issued by the guest operating system to the remote devices by referring to a management table for managing correspondence between the guest operating system and the remote devices by using identification information that does not change dependently on start-up states of the remote devices; and selecting a reply to be transferred to the guest operating system, out of replies sent back by the remote devices by referring to the management table is provided. 
         [0021]    According to a third aspect, a computer-readable program recording medium for recording a program executed by a computer, which is arranged in a computer system including a bus extending means which constructs a virtual bus to one or more remote devices connected via a network and an executing means which configures a virtual hardware environment and executes a host operating system for causing a guest operating system to control the remote devices, the computer executing: a process of transferring a scan instruction issued by the guest operating system to the remote devices by referring to a management table for managing correspondence between the guest operating system and the remote devices by using identification information that does not change dependently on start-up states of the remote devices; and a process of selecting a reply to be transferred to the guest operating system, out of replies sent back by the remote devices by referring to the management table. 
       Advantageous Effects of Invention 
       [0022]    According to the present invention, it is possible to improve easiness of connection between the guest OS and the remote device in a configuration combining the pass-through technology and the bus extension technology. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  is a diagram showing a configuration of an exemplary embodiment of the present invention. 
           [0024]      FIG. 2  is a diagram showing a configuration of a first exemplary embodiment of the present invention. 
           [0025]      FIG. 3  is a block diagram showing a configuration of a remote apparatus of the first exemplary embodiment of the present invention. 
           [0026]      FIG. 4  is a block diagram showing a configuration of a computer system of the first exemplary embodiment of the present invention. 
           [0027]      FIG. 5  is a diagram showing an example of entry composition of a device management table which the computer system of the first exemplary embodiment of the present invention includes. 
           [0028]      FIG. 6  is a diagram showing a setting example of the device management table which the computer system of the first exemplary embodiment of the present invention includes. 
           [0029]      FIG. 7  is a flowchart showing an operation of the remote apparatus of the first exemplary embodiment of the present invention. 
           [0030]      FIG. 8  is a flowchart showing an operation of the computer system of the first exemplary embodiment of the present invention. 
           [0031]      FIG. 9  is a block diagram for describing a configuration of a virtualized computer. 
           [0032]      FIG. 10  is a block diagram for describing a configuration of a computer system which uses the bus extension technology. 
           [0033]      FIG. 11  is a reference diagram for describing a problem of the present invention. 
           [0034]      FIG. 12  is a diagram showing a configuration of a modification of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    Firstly, an outline of an exemplary embodiment of the present invention will be described with reference to drawings. Here, a reference sign of the drawing, which is added supplementally to the outline, is assigned to each element for the sake of convenience as an example helpful in understanding, and it is not intended to limit the present invention to an aspect shown in the drawing. 
         [0036]    The present invention can be realized by a computer system including a bus extending unit  140 A, an OS executing unit  120 A and a communication control unit  142 A, which are shown in  FIG. 1 , in the exemplary embodiment. The bus extending unit  140 A constructs a virtual bus to one or more remote devices which are connected through a network. The OS executing unit  120 A constructs a virtual hardware environment and executes a host OS  110  which causes each of guest OSs  101 - 10   n  to control the remote devices. The communication control unit  142 A controls communication between the guest OSs  101 - 10   n  and the remote devices. 
         [0037]    More specifically, the communication control unit  142 A controls communication between the guest OSs  101 - 10   n  and the remote devices by referring to a management table  141 A, which manages correspondence between the guest OSs  101 - 10   n  and the remote devices by using identification information not changing dependently on start-up states of the remote devices. 
         [0038]    As mentioned above, the computer system of the present exemplary embodiment manages the correspondence between the guest OS and the remote device by using the identification information which does not change dependently on the start-up states of the remote devices. As a result, it is possible that the guest OS designates the remote device with accuracy even in the case that some remote devices are not started up. 
       FIRST EXEMPLARY EMBODIMENT 
       [0039]    Next, a first exemplary embodiment of the present invention will be described in detail with reference to drawings. Here, a reference sign of the drawing, which will be supplementally added hereinafter, is exemplified just for a help in understanding, and it is not intended to limit the present invention to an aspect shown in the drawing. 
         [0040]      FIG. 2  is a diagram showing a configuration of the first exemplary embodiment of the present invention. As shown in  FIG. 2 , a computer system  100  and a remote apparatus  200  are connected each other through a network  300 . 
         [0041]      FIG. 3  is a block diagram exemplifying a configuration of the remote apparatus  200 . Hereinafter, the configuration of the remote apparatus  200  will be described in detail with reference to  FIG. 3 . 
         [0042]    The remote apparatus  200  includes a communication unit  210 , a downstream bridge (bus extending unit (remote))  220  and n (n is an integer equal to or larger than 1) remote devices  231  to  23   n.  Each of the remote devices  231  to  23   n  may a device which is connected with a system bus. Each of the remote devices  231  to  23   n  is, for example, a hard disk, a network interface card or the like which can be directly connected with the bus. 
         [0043]    The downstream bridge (bus extending unit (remote))  220  forms a pair with an upstream bridge (bus extending unit (host)) which is arranged on the computer system side and will be described later. The downstream bridge (bus extending unit (remote))  220  executes de-capsulation for data which should be transmitted toward the remote devices  231  to  23   n,  and executes encapsulation for data on the bus which are transmitted by the remote devices  231  to  23   n.  Moreover, the downstream bridge (bus extending unit (remote))  220  may have a function such as transmission rate control, retransmission control which is used when a packet is lost on the network, or the like. 
         [0044]    The communication unit  210  is an interface unit for communicating through the network  300 . Ethernet®, InfiniBand, or the like is an example of the network  300 . However, these networks are only examples. The network  300  has no limitation as far as the bus extension technology, which is described as Background Art, is applicable between the computer system  100  and the remote apparatus  200 . 
         [0045]    The communication unit  210  of the remote apparatus  200  is assigned an address (for example, MAC (Media Access Control) address) which can be identified uniquely on the network  300 . In the present exemplary embodiment, the MAC address is used as identification information which does not change dependently on a start-up states of the remote devices (described in detail later). 
         [0046]    The downstream bridge (bus extending unit (remote))  220  transmits heartbeat (HB: Heart Beat) information intermittently towards the network  300  via the communication unit  210 . Generally, the heartbeat information is broadcasted. However, when identifying an address of the host in advance, the downstream bridge (bus extending unit (remote))  220  may transmit the heartbeat information only to the host which is identified by the address. The heartbeat information includes information on the remote devices  231 - 23   n  which are arranged in the remote apparatus  200 , and the MAC address of communication unit  210 . 
         [0047]      FIG. 4  is a block diagram showing a configuration of the computer system  100  of the present exemplary embodiment. Referring to  FIG. 4 , the computer system  100  includes a CPU  120 , a peripheral device  130 , an upstream bridge (bus extending unit (host))  140  and a communication unit  150 . Moreover, the CPU  120  executes various programs including the host OS  110  and the guest OS  101 - 10   n  (n is an integer equal to or larger than 1). 
         [0048]    The peripheral device  130  is a device which can be connected with the system bus. 
         [0049]    The upstream bridge (bus extending unit (host))  140  is paired with the downstream bridge (bus extending unit (remote))  220  of the remote apparatus  200 . When transmitting a data packet, which flows on the bus, toward the network  300 , the upstream bridge (bus extending unit (host))  140  encapsulates the data packet and transmits the encapsulated data packet. Meanwhile, when receiving the encapsulated network packet from the network  300 , the upstream bridge (bus extending unit (host))  140  resolves the encapsulation (decapsulates) and transfers the data packet to the guest OSs  101 - 10   n  or the system bus. 
         [0050]    As a communication protocol which is used between the upstream bridge (bus extending unit (host))  140  and the downstream bridge (bus extending unit (remote))  220 , TCP (Transmission Control Protocol)/IP (Internet Protocol) or the like is applicable. However, TCP/IP is an example, and the communication protocol is not limited to TCP/IP. 
         [0051]    The computer system  100  includes the host OS  110 . The host OS  110  provides a virtual hardware environment to the guest OS s  101 - 10   n  executing I/O processes with the devices (remote device  231  to  23   n ) respectively which are included in the remote apparatus  200 . The host OS  110  controls the upstream bridge (bus extending unit (host))  140  to encapsulate an I/O instruction which each of the guest OSs  101 - 10   n  issues, and to transmit the encapsulated I/O instruction toward the remote apparatus  200  as a packet. Here, in the case that it is unnecessary to distinguish each of the guest OSs  101 - 10   n,  the guest OSs  101 - 10   n  are collectively denoted as a ‘guest OS  10   x’.    
         [0052]    When receiving a reply packet, which is a reply to the transmitted packet, from the remote apparatus  200 , the upstream bridge (bus extending unit (host))  140  decapsulates the reply packet to extract data, and transfers the extracted data to the guest OS  10   x.    
         [0053]    The communication unit  150  is an interface for connection with a network  600  which is different from the network  300  connected with the communication unit  210  of the remote apparatus  200 . 
         [0054]    The guest OS  10   x  includes device drivers for controlling the remote devices  231  to  23   n  of the remote apparatus  200  respectively. Here, it is not necessary that the guest OS  10   x  includes the device drivers corresponding to all of the remote devices  231  to  23   n,  and the gust OS  10   x  may include the device drivers which can control required remote devices out of the remote device  231  to  23   n.    
         [0055]    The upstream bridge (bus extending unit (host))  140  of the present exemplary embodiment includes a device management table  141 , a scan result checking unit  142  and a network adaptor  143 . 
         [0056]      FIG. 5  is a diagram showing composition of an entry of the device management table  141 . The device management table  141  is a table which designates a combination of the guest OS and the remote device in advance. In the case of the example shown in  FIG. 5 , the device management table  141  can store an entry including an MAC address, a vendor ID, a device ID and a corresponding guest OS which are associated each other. 
         [0057]    The Vendor ID and the device ID are numbers to identify the remote devices  231  to  23   n  which exist in the remote apparatus  200 . When the devices of the vendor IDs or the device IDs belong to the same kind, the vendor IDs or the device IDs have the same ID. The MAC address is a MAC address of the communication unit of the remote apparatus including the device. The corresponding guest OS indicates an identifier of the guest OS combined with the remote device, which is designated by the entry, out of the remote devices  231  to  23   n  of the remote apparatus  200  (the guest OS has a right to access the remote device out of the remote devices  231  to  23   n ). 
         [0058]      FIG. 6  shows a specific setting example of the device management table  141 . In the example of  FIG. 6 , information on a remote device of a remote apparatus  200  whose MAC address of the communication unit  210  is ‘AA::AA’ and a remote device of a remote apparatus  200  whose MAC address of the communication unit  210  is ‘BB::BB’ is set. 
         [0059]    For example, a remote device, whose the MAC address, the vendor ID and the device ID are ‘AA::AA’, ‘Oxabcd’ and ‘0x1234’ respectively, is assigned a guest OS-A as the corresponding guest OS. Similarly, a remote device, whose the MAC address, the vendor ID and the device ID are ‘BB::BB’, ‘Oxabcd’ and ‘0x1234’ respectively, is assigned a guest OS-B as the corresponding guest OS. As mentioned above, even if the vendor IDs have the same ID or the device IDs have the same ID, it is possible to distinguish the devices by using the MAC address, and consequently it is possible to associate the vendor ID and the device ID with the guest OS. 
         [0060]    Moreover, another remote device, whose the MAC address, the vendor ID and the device ID are ‘AA::AA’, ‘0xcdef’ and ‘0x9876’ respectively, is assigned the gust  05 -B as the corresponding guest OS. 
         [0061]    In the case of the setting shown in  FIG. 6 , when a device scan instruction is issued by the guest OS-A, a scan result (reply) of the remote device, whose the vendor ID, and the device ID are ‘0xcdef’ and ‘0x9876’ respectively, reaches the upstream bridge (bus extending unit (host))  140 . When, by referring to the device management table  141  shown in  FIG. 6 , the scan result checking unit  142  of the upstream bridge (bus extending unit (host)) checks that the combination is not permitted, the scan result checking unit  142  discards the scan result (reply). At this time, the scan result checking unit  142  sends back error information to the guest OS-A. Therefore, the guest OS-A does not recognize the device whose the vendor ID and the device ID are ‘0xcdef’ and ‘0x9876’ respectively. 
         [0062]    Here, in the examples shown in  FIG. 5  and  FIG. 6 , the MAC address of the communication unit  210  of the remote apparatus is used as the identification information which does not change dependently on the start-up states of the remote devices. However, the identification information which does not change dependently on the start-up states of the remote devices is not limited to the MAC address, and identification information which is different from the MAC address and which is assigned to the remote apparatus may be used in place of the MAC address. 
         [0063]    Here, the scan result checking unit  142  of the computer system  100  shown in  FIG. 4  can be realized by a computer program which works on the host OS  110 . Moreover, the device management table  141  and the scan result checking unit  142  can be incorporated in a virtual BIOS (Basic Input/Output System) which the host OS  110  provides.  FIG. 12  is a diagram showing a configuration of a computer system  100   a  according to a configuration of a modification of the exemplary embodiment. In the case of the example shown in  FIG. 12 , a device management table  141   a  and a scan result checking unit  142   a  are incorporated in a virtual BIOS  111  which the host OS  110  provides. 
         [0064]    Next, an operation of the exemplary embodiment will be described in detail with reference to drawings. In the following explanation, it is assumed that the remote apparatus  200  is started up before the start-up of the computer system  100 .  FIG. 7  is a flowchart showing an operation executed after the start-up of the remote apparatus of the present exemplary embodiment. Referring to  FIG. 7 , firstly, the communication unit  210  and the downstream bridge (bus extending unit (remote))  220  of the remote apparatus  200  are started up (Step S 001 ). At this point of time, each of the remote devices  231  to  23   n  receives electric power to make an own power source voltage turn ON, but only initialization of the remote devices  231  to  23   n  by hardware is executed, and initialization by the device driver is not executed. 
         [0065]    Next, the downstream bridge (bus extending unit (remote))  220  starts intermittent transmission of the packet, in which the heartbeat information is embedded, toward the network  300  via the communication unit  210  (Step S 002 ). In the case that the host OS  110  of the computer system  100  is not started up, the heartbeat information is discarded on the network  300  without being received. 
         [0066]    Next, an operation of the computer system  100 , which is executed after the start-up of the computer system  100 , will be described.  FIG. 8  is a flowchart showing the operation of the computer system  100  of the present exemplary embodiment. When the computer system  100  is started up, the host OS  110  is firstly started up (Step S 101 ). The host OS  110  initializes the peripheral device  130  and the communication unit  150  which are included by the computer system  100 , and scans the remote devices  231  to  23   n  through the upstream bridge (bus extending unit (host))  140  at the same time. Accordingly, the communication unit  150  of the computer system  100  becomes available. However, it is not necessary to initialize the remote devices  231  to  23   n  of the remote apparatus  200 . 
         [0067]    Since the remote apparatus  200  has been already started up as mentioned above, the upstream bridge (bus extending unit (host))  140  receives the heartbeat information which is transmitted by the remote apparatus  200  (Step S  102 ). By receiving the heartbeat information, the upstream bridge (bus extending unit (host))  140  can recognize the remote devices  231  to  23   n  which are connected with the remote apparatus  200  and can acquire information on the remote devices  231  to  23   n  which are connected with the remote apparatus  200 . Moreover, the upstream bridge (bus extending unit (host))  140  acquires the MAC address of the communication unit  210  of the remote apparatus  200  based on the heartbeat information. 
         [0068]    Next, when any guest OS  10   x  is started up, the guest OS  10   x  issues an instruction for scanning the devices (step S 103 ). The upstream bridge (bus extending unit (host))  140 , by retrieving the device management table  141 , designates the MAC address of the communication unit  210  of the remote apparatus  200  having the guest OS  10   x  which is permitted, and issues the device scan instruction. Since the MAC address of the communication unit  210  is a uniquely determined ID as mentioned above, even if some of the plural remote apparatuses  200  are not started up, a combination of the guest OS  10   x  and the remote apparatus  200  does not change. 
         [0069]    The remote apparatus  200  which has received the device scan instruction replies classification of the device to a source of the device scan instruction. As the reply on the classification of the device, a combination of ‘vendor ID and device ID’ shown in  FIG. 5  and  FIG. 6  is exemplified. Apparently, ‘vendor ID and device ID’ is a mere example of the reply. The remote apparatus  200  may transmit device classification information instead of ‘vendor ID and device ID’, and may add other pieces of information. 
         [0070]    When the scan result checking unit  142  of the upstream bridge (bus extending unit (host))  140  receives the reply on the classification of the device, the scan result checking unit  142  refers to the device management table and determines whether or not a combination of the classification of the device with the guest OS  10   x  is permitted (Steps S 104  to S 105 ). In the case that a combination of the classification of the device and the guest OS  10   x  is the permitted combination (Yes in Step S 105 ), the scan result checking unit  142  sends back the scan result to the guest OS  10   x  (Step S 106 ). The guest OS  10   x  which has detected the remote device initializes the remote device by use of the device driver, and accesses the remote device if necessary (Step S 108 ). 
         [0071]    On the other hand, in the case that a combination of the classification of the device and the guest OS  10   x  is not the permitted combination (No in Step S 105 ), the scan result checking unit  142  discards the scan result, and sends back the error information (the device is not detected) to the guest OS  10   x  (Step S 107 ). 
         [0072]    As mentioned above, the present exemplary embodiment achieves a remarkable effect that, even if some out of one or more remote apparatuses  200  are not started up, it is unnecessary to change the method for designating the remote apparatus  200  which is assigned to the guest OS  10   x.  The effect is achieved by using the identifier (MAC address) specific to the communication unit  210  of the remote apparatus  200  as the identification information for identifying the remote device. Moreover, even if the upstream bridge (bus extending unit (host))  140   a  does not include the device management table  141  and the scan result checking unit  142 , the configuration shown in  FIG. 12  can achieve the same effect. 
         [0073]    While each exemplary embodiment of the present invention has been described, the present invention is not limited to the above-mentioned exemplary embodiments, and it is possible to add a further modification, replacement and adjustment without departing from the basic technical concept of the present invention. For example, the configuration of the network, the configuration of each element and the description form of the message, which are shown in each drawing, are examples for assisting in understanding the present invention, and the present invention is not limited to the configuration shown in each the drawing. 
         [0074]    Finally, a preferred form of the present invention will be summarized. 
       First Form 
       [0075]    (refer to the computer system according to the first aspect) 
       Second Form 
       [0076]    The computer system of the first form, wherein the communication control means functions as a scan result checking means which discards, out of replies sent back by the remote devices in reply to a scan instruction issued by a guest operating system, a reply sent back by a remote device not associated with the guest operating system issuing the scan instruction, by referring to the management table. 
       Third Form 
       [0077]    The computer system of the first or the second form, wherein, in a case where the guest operating system issues the scan instruction, the communication control means rewrites a destination of the scan instruction to a MAC address which is embedded in heartbeat information received from a remote apparatus including the remote device and transfers the scan instruction using the MAC address as the identification information. 
       Fourth Form 
       [0078]    The computer system of any one of the first to the third forms, the remote devices are PCI-Express devices subject to the PCI-Express protocol, and 
         [0079]    the bus extending means constructs the virtual bus to the remote devices by executing encapsulation based on the PCI-Express protocol. 
       Fifth Form 
       [0080]    (refer to the connection management method for the remote device according to the above-mentioned second aspect) 
       Sixth Form 
       [0081]    The connection management method for the remote device of the fifth form, wherein 
         [0082]    using a MAC address which is embedded in heartbeat information received from a remote apparatus including the remote devices as the identification information that does not change dependently on the start-up states of the remote devices, and 
         [0083]    the communication control means rewrites a destination of the scan instruction to the MAC address and transfers the scan instruction in a case where the guest operating system issues the scan instruction. 
       Seventh Form 
       [0084]    The connection management method for the remote device of the fifth or the sixth form, wherein 
         [0085]    the remote devices are PCI-Express devices subject to the PCI-Express protocol, 
         [0086]    the method further comprising executing communication between the guest operating system and the remote devices through the virtual bus which is constructed to the remote devices by executing encapsulation based on the PCI-Express protocol. 
       Eighth Form 
       [0087]    (refer to the program according to the above-mentioned third aspect) 
         [0088]    It is to be noted that the eighth form can develop to the second to the fourth forms as well as the first form. 
         [0089]    It is assumed that each disclosure of the above-mentioned patent documents is incorporated in the present DESCRIPTION by reference. Within the scope of all disclosures (including Claims) of the present invention and based on the basic technical concept, it is possible to change and adjust the exemplary embodiments or the examples. In addition, it is possible to variously combine various disclosed-elements (including each element of each Claim, each exemplary embodiment, each example, each drawing and the like) and to select the disclosed element within the scope of the disclosures of the present invention. That is, it is apparent that the present invention includes various changes and modifications which a person skilled in the art can achieve based on all of the disclosures, which includes Claims, and the technical concept. Particularly, regarding the range of numerical value which has been described by the present DESCRIPTION, even if there is no specific description of any numerical value or any small range which is within the range of numerical value, the numerical value or the small range should be interpreted as being specifically described. 
         [0090]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-121450, filed on Jun. 12, 2014, the disclosure of which is incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0091]      100 ,  100   a,    100 A, X 100  and X 500  computer system 
         [0092]      101 - 10   n,    10   x,  and X 101 -X 10   n  guest OS 
         [0093]      110  and X 110  host OS 
         [0094]      111  virtual BIOS 
         [0095]      120  and X 120  CPU 
         [0096]      120 A OS executing unit 
         [0097]      130  peripheral device 
         [0098]      140 , X 140  and  140   a  upstream bridge (bus extending unit (host)) 
         [0099]      140 A bus extending unit 
         [0100]      141  and  141   a  device management table 
         [0101]      141 A management table 
         [0102]      142  and  142   a  scan result checking unit 
         [0103]      142 A communication control unit 
         [0104]      143 , X 141  and X 421  network adaptor 
         [0105]      150 , X 150  and X 510  communication unit 
         [0106]      200 , X 20 A, X 20 B and X 200  remote apparatus 
         [0107]      210  communication unit 
         [0108]      220  and X 420  downstream bridge (bus extending unit (remote)) 
         [0109]      231  to  23   n  remote device 
         [0110]      300  and  600  network 
         [0111]    X 130  peripheral equipment 
         [0112]    X 131  device B 
         [0113]    X 410  Ethernet switch 
         [0114]    X 600  switch