Abstract:
In a distributed computer system having multiple root nodes, a challenge protocol is provided, for use in determining or confirming the root node in which a PCI Configuration Manager (PCM) actually resides. This node is referred to as the master node. The challenge procedure is activated whenever the identity of the PCM, which is determined by the root node in which it resides, appears to be uncertain. The challenge procedure resolves this uncertainty, and enables the PCM to continue to configure routings throughout the system. In a useful embodiment, a method is directed to a distributed computer system of the above type which is further provided with PCI switches and with adapters that are available for sharing by different nodes. The method includes the steps of selecting a first one of the root nodes to be master root node, and operating the first root node to query the configuration space of a particular one of the PCI switches. The method further includes detecting information indicating that a second root node is considered to be the master root node for the particular switch. A challenge protocol is implemented in response to this detected information, to seek confirmation that the first root node is the master root node. The configuration space querying procedure is continued if the first root node is confirmed to be the master root node, and is otherwise aborted.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention disclosed and claimed herein generally pertains to a method and related apparatus for data transfer between multiple root nodes and PCI adapters, through an input/output (I/O) switched-fabric bus. More particularly, the invention pertains to a method of the above type wherein different root nodes may be routed through the I/O fabric to share the same adapter, and a single control, used to configure the routing for all root nodes, resides in one of the nodes. Even more particularly, the invention pertains to a method of the above type wherein a challenge procedure is provided, to resolve any uncertainty as to which node is serving as the control node. 
   2. Description of the Related Art 
   As is well known by those of skill in the art, PCI Express (PCI-E) is widely used in computer systems to interconnect host units to adapters or other components, by means of an I/O switched-fabric bus or the like. However, PCI-E currently does not permit sharing of PCI adapters in topologies where there are multiple hosts with multiple shared PCI buses. As a result, even though such sharing capability could be very valuable when using blade clusters or other clustered servers, adapters for PCI-E and secondary networks (e.g., FC, IB, Enet) are at present generally integrated into individual blades and server systems. Thus, such adapters cannot be shared between clustered blades, or even between multiple roots within a clustered system. 
   In an environment containing multiple blades or blade clusters, it can be very costly to dedicate a PCI adapter for use with only a single blade. For example, a 10 Gigabit Ethernet (10 GigE) adapter currently costs on the order of $6,000. The inability to share these expensive adapters between blades has, in fact, contributed to the slow adoption rate of certain new network technologies such as 10 GigE. Moreover, there is a constraint imposed by the limited space available in blades to accommodate PCI adapters. This problem of limited space could be overcome if a PC network was able to support attachment of multiple hosts to a single PCI adapter, so that virtual PCI I/O adapters could be shared between the multiple hosts. 
   In a distributed computer system comprising a multi-host environment or the like, the configuration of any portion of an I/O fabric that is shared between hosts, or other root nodes, cannot be controlled by multiple hosts. This is because one host might make changes that affect another host. Accordingly, to achieve the above goal of sharing a PCI adapter amongst different hosts, it is necessary to provide a central management mechanism of some type. This management mechanism is needed to configure the routings used by PCI switches of the I/O fabric, as well as by the root complexes, PCI adapters and other devices interconnected by the PCI switches. 
   It is to be understood that the term “root node” is used herein to generically describe an entity that may comprise a computer host CPU set or the like, and a root complex connected thereto. The host set could have one or multiple discrete CPU&#39;s. However, the term “root node” is not necessarily limited to host CPU sets. The term “root complex” is used herein to generically describe structure in a root node for connecting the root node and its host CPU set to the I/O fabric. 
   In one very useful approach, a particular designated root node includes a component which is the PCI Configuration Master (PCM) for the entire multi-host system. The PCM configures all routings through the I/O fabric, for all PCI switches, root complexes and adapters. However, in a PCI switched-fabric, multiple fabric managers are allowed. Moreover, any fabric manager can plug into any root switch port, that is, the port of a PCI switch that is directly connected to a root complex. As a result, when a PCM of the above type is engaged in configuring a route through a PCI fabric, it will sometimes encounter a switch that appears to be controlled by a fabric manager other than the PCM, residing at a root node other than the designated node. Accordingly, it is necessary to provide a challenge procedure, to determine or affirm which root node actually contains the controlling fabric configuration manager. 
   SUMMARY OF THE INVENTION 
   The invention generally provides a challenge procedure or protocol for determining the root node in which the PCI Configuration Master or Manager actually resides, in a multi-host system of the above type. This node is referred to as the master node. The challenge procedure is activated whenever the identity of the PCM, determined by the root node containing the PCM, appears to be uncertain. The challenge procedure resolves this uncertainty, and enables the PCM to continue to configure routings throughout the system. In one useful embodiment, the invention is directed to a method for a distributed computer system provided with multiple root nodes, and further provided with one or more PCI switches and with adapters or other components that are available for sharing by different nodes. The method includes the steps of selecting a first one of the root nodes to be the master root node for the system, and operating the first root node to implement a procedure whereby the first root node queries the configuration space of a particular one of the PCI switches. The method further includes detecting information indicating that a second root node, rather than the first root node, is considered to be the master root node for the particular switch. A challenge procedure is implemented in response to this detected information, in an effort to confirm that the first root node is in fact the master root node for the system. The configuration space querying procedure is then continued, if the first root node is confirmed to be the master root node. Otherwise, the querying procedure is aborted so that corrective action can be taken. Usefully, when the PCM is performing PCI configuration, all the root nodes are in a quiescent state. After the switched-fabric has been configured, the PCM writes the configuration information into the root switches, and then enables each of the root ports to access its configuration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram showing a generic distributed computer system in which an embodiment of the invention may be implemented. 
       FIG. 2  is a block diagram showing an exemplary logical partitioned platform in the system of  FIG. 1 . 
       FIG. 3  is a block diagram showing a distributed computer system provided with multiple hosts and respective PCI family components that are collectively operable in accordance with an embodiment of the invention. 
       FIG. 4  is a schematic diagram depicting a PCI configuration space adapted for use with an embodiment of the invention. 
       FIG. 5  is a schematic diagram showing an information space for each of the host sets of the system of  FIG. 3 . 
       FIG. 6  is a schematic diagram showing components of a fabric table constructed by the PCM to provide a record of routings that have been configured or set up. 
       FIG. 7  is a flow chart depicting steps carried out by the PCM in constructing the table of  FIG. 6 , including steps for an embodiment of the invention. 
       FIG. 8  is a flow chart depicting a challenge protocol in accordance with the embodiment of  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a distributed computer system  100  in which a preferred embodiment of the present invention may be practiced. The distributed computer system  100  takes the form of multiple root complexes (RCs)  110 ,  120 ,  130 ,  140  and  142 , respectively connected to an I/O fabric  144  through I/O links  150 ,  152 ,  154 ,  156  and  158 , and to the memory controllers  108 ,  118 ,  128  and  138  of the root nodes (RNs)  160 - 166 . The I/O fabric is attached to I/O adapters (IOAs)  168 - 178  through links  180 - 194 . The IOAs may be single function, such as IOAs  168 - 170  and  176 , or multiple function, such as IOAs  172 - 174  and  178 . Moreover, respective IOAs may be connected to the I/O fabric  144  via single links, such as links  180 - 186 , or with multiple links for redundancy, such as links  188 - 194 . 
   The RCs  110 ,  120 , and  130  are integral components of RN  160 ,  162  and  164 , respectively. There may be more than one RC in an RN, such as RCs  140  and  142  which are both integral components of RN  166 . In addition to the RCs, each RN consists of one or more Central Processing Units (CPUs)  102 - 104 ,  112 - 114 ,  122 - 124  and  132 - 134 , memories  106 ,  116 ,  126  and  128 , and memory controllers  108 ,  118 ,  128  and  138 . The memory controllers respectively interconnect the CPUs, memory, and I/O RCs of their corresponding RNs, and perform such functions as handling the coherency traffic for respective memories. 
   RN&#39;s may be connected together at their memory controllers, such as by a link  146  extending between memory controllers  108  and  118  of RNs  160  and  162 . This forms one coherency domain which may act as a single Symmetric Multi-Processing (SMP) system. Alternatively, nodes may be independent from one another with separate coherency domains as in RNs  164  and  166 . 
     FIG. 1  further shows a PCI Configuration Manager (PCM)  148  incorporated into one of the RNs, such as RN  160 , as an integral component thereof. The PCM configures the shared resources of the I/O fabric and assigns resources to the RNs. 
   It is to be understood that any one of the root nodes  160 - 166  could support the PCM. However, there must be only one PCM, to configure all routes and assign all resources, throughout the entire system  100 . Clearly, significant uncertainties could develop if it appeared that there was more than one PCM in system  100 , with each PCM residing in a different root node. Accordingly, embodiments of the invention are provided, first to determine that an uncertain condition regarding the PCM exists, and to then resolve the uncertainty. 
   In a very useful embodiment, a challenge protocol is operable to recognize that a PCI switch, included in the switched-fabric of the system, appears to be under the control of a PCM that is different from the PCM currently in control of the system. Upon recognizing this condition, the challenge protocol will either confirm that the current PCM has control over the switch, or else will abort configuration of the switch. This challenge protocol or procedure is described hereinafter in further detail, in connection with  FIGS. 7 and 8 . 
   Distributed computing system  100  may be implemented using various commercially available computer systems. For example, distributed computing system  100  may be implemented using an IBM eServer iSeries Model 840 system available from International Business Machines Corporation. Such a system may support logical partitioning using an OS/400 operating system, which is also available from International Business Machines Corporation. 
   Those of ordinary skill in the art will appreciate that the hardware depicted in  FIG. 1  may vary. For example, other peripheral devices, such as optical disk drives and the like, also may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural limitations with respect to the present invention. 
   With reference to  FIG. 2 , a block diagram of an exemplary logical partitioned platform  200  is depicted in which the present invention may be implemented. The hardware in logically partitioned platform  200  may be implemented as, for example, data processing system  100  in  FIG. 1 . Logically partitioned platform  200  includes partitioned hardware  230 , operating systems  202 ,  204 ,  206 ,  208  and hypervisor  210 . Operating systems  202 ,  204 ,  206  and  208  may be multiple copies of a single operating system, or may be multiple heterogeneous operating systems simultaneously run on platform  200 . These operating systems may be implemented using OS/400, which is designed to interface with a hypervisor. Operating systems  202 ,  204 ,  206  and  208  are located in partitions  212 ,  214 ,  216  and  218 , respectively. Additionally, these partitions respectively include firmware loaders  222 ,  224 ,  226  and  228 . When partitions  212 ,  214 ,  216  and  218  are instantiated, a copy of open firmware is loaded into each partition by the hypervisor&#39;s partition manager. The processors associated or assigned to the partitions are then dispatched to the partitions&#39; memory to execute the partition firmware. 
   Partitioned hardware  230  includes a plurality of processors  232 - 238 , a plurality of system memory units  240 - 246 , a plurality of input/output (I/O) adapters  248 - 262 , and a storage unit  270 . Partition hardware  230  also includes service processor  290 , which may be used to provide various services, such as processing of errors in the partitions. Each of the processors  232 - 238 , memory units  240 - 246 , NVRAM  298 , and I/O adapters  248 - 262  may be assigned to one of multiple partitions within logically partitioned platform  200 , each of which corresponds to one of operating systems  202 ,  204 ,  206  and  208 . 
   Partition management firmware (hypervisor)  210  performs a number of functions and services for partitions  212 ,  214 ,  216  and  218  to create and enforce the partitioning of logically partitioned platform  200 . Hypervisor  210  is a firmware implemented virtual machine identical to the underlying hardware. Hypervisor software is available from International Business Machines Corporation. Firmware is “software” stored in a memory chip that holds its content without electrical power, such as, for example, read-only memory (ROM), programmable ROM (PROM), electrically erasable programmable ROM (EEPROM), and non-volatile random access memory (NVRAM). Thus, hypervisor  210  allows the simultaneous execution of independent OS images  202 ,  204 ,  206  and  208  by virtualizing all the hardware resources of logically partitioned platform  200 . 
   Operation of the different partitions may be controlled through a hardware management console, such as hardware management console  280 . Hardware management console  280  is a separate distributed computing system from which a system administrator may perform various functions including reallocation of resources to different partitions. 
   In an environment of the type shown in  FIG. 2 , it is not permissible for resources or programs in one partition to affect operations in another partition. Moreover, to be useful, the assignment of resources needs to be fine-grained. For example, it is often not acceptable to assign all IOAs under a particular PHB to the same partition, as that will restrict configurability of the system, including the ability to dynamically move resources between partitions. 
   Accordingly, some functionality is needed in the bridges that connect IOAs to the I/O bus so as to be able to assign resources, such as individual IOAs or parts of IOAs to separate partitions; and, at the same time, prevent the assigned resources from affecting other partitions such as by obtaining access to resources of the other partitions. 
   Referring to  FIG. 3 , there is shown a distributed computer system  300  that includes a more detailed representation of the I/O switched-fabric  144  depicted in  FIG. 1 . More particularly, to further illustrate the concept of a PCI family fabric that supports multiple root nodes through the use of multiple switches, fabric  144  is shown in  FIG. 3  to comprise a plurality of PCIe switches (or PCI family bridges)  302 ,  304  and  306 .  FIG. 3  further shows switches  302 ,  304  and  306  provided with ports  308 - 314 ,  316 - 324  and  326 - 330 , respectively. The switches  302  and  304  are referred to as multi-root aware switches, for reasons described hereinafter. It is to be understood that the term “switch”, when used herein by itself, may include both switches and bridges. The term “bridge” as used herein generally pertains to a device for connecting two segments of a network that use the same protocol. 
   Referring further to  FIG. 3 , there are shown host CPU sets  332 ,  334  and  336 , each containing a single or a plurality of system images (SIs). Thus, host  332  contains system image SI  1  and SI  2 , host  334  contains system image SI  3 , and host  336  contains system images SI  4  and SI  5 . It is to be understood that each system image is equivalent or corresponds to a partition, as described above in connection with  FIG. 2 . Each of the host CPU sets has an associated root complex as described above, through which the system images of respective hosts interface with or access the I/O fabric  144 . More particularly, host sets  332 - 336  are interconnected to RCs  338 - 342 , respectively. Root complex  338  has ports  344  and  346 , and root complexes  340  and  342  each has only a single port, i.e. ports  348  and  350 , respectively. Each of the host CPU sets, together with its corresponding root complex, comprises an example or instance of a root node, such as RNs  160 - 166  shown in  FIG. 1 . Moreover, host CPU set  332  is provided with a PCM  370  that is similar or identical to the PCM  148  of  FIG. 1 . 
     FIG. 3  further shows each of the RCs  338 - 342  connected to one of the ports  316 - 320 , which respectively comprise ports of multi-root aware switch  304 . Each of the multi-root aware switches  304  and  302  provides the capability to configure a PCI family fabric such as I/O fabric  144  with multiple routings or data paths, in order to accommodate multiple root nodes. 
   Respective ports of a multi-root aware switch, such as switches  302  and  304 , can be used as upstream ports, downstream ports, or both upstream and downstream ports. Generally, upstream ports are closer to the RC. Downstream ports are further from RC. Upstream/downstream ports can have characteristics of both upstream and downstream ports. In  FIG. 3  ports  316 ,  318 ,  320 ,  326  and  308  are upstream ports. Ports  324 ,  312 ,  314 ,  328  and  330  are downstream ports, and ports  322  and  310  are upstream/downstream ports. 
   The ports configured as downstream ports are to be attached or connected to adapters or to the upstream port of another switch. In  FIG. 3 , multi-root aware switch  302  uses downstream port  312  to connect to an I/O adapter  352 , which has two virtual I/O adapters or resources  354  and  356 . Similarly, multi-root aware switch  302  uses downstream port  314  to connect to an I/O adapter  358 , which has three virtual I/O adapters or resources  360 ,  362  and  364 . Multi-root aware switch  304  uses downstream port  324  to connect to port  326  of switch  306 . Multi-root aware switch  304  uses downstream ports  328  and  330  to connect to I/O adapter  366 , which has two virtual I/O adapters or resources  353  and  351 , and to I/O adapter  368 , respectively. 
   Each of the ports configured as an upstream port is used to connect to one of the root complexes  338 - 342 . Thus,  FIG. 3  shows multi-root aware switch  302  using upstream port  308  to connect to port  344  of RC  338 . Similarly, multi-root aware switch  304  uses upstream ports  316 ,  318  and  320  to respectively connect to port  346  of root complex  338 , to the single port  348  of RC  340 , and to the single port  350  of RC  342 . 
   The ports configured as upstream/downstream ports are used to connect to the upstream/downstream port of another switch. Thus,  FIG. 3  shows multi-root aware switch  302  using upstream/downstream port  310  to connect to upstream/downstream port  322  of multi-root aware switch  304 . 
   I/O adapter  352  is shown as a virtualized I/O adapter, having its function  0  (F 0 ) assigned and accessible to the system image SI  1 , and its function  1  (F 1 ) assigned and accessible to the system image SI  2 . Similarly, I/O adapter  358  is shown as a virtualized I/O adapter, having its function  0  (F 0 ) assigned and assessible to SI  3 , its function  1  (F 1 ) assigned and accessible to SI  4  and its function  3  (F 3 ) assigned to SI  5 . I/O adapter  366  is shown as a virtualized I/O adapter with its function F 0  assigned and accessible to SI  2  and its function F 1  assigned and accessible to SI  4 . I/O adapter  368  is shown as a single function I/O adapter assigned and accessible to SI  5 . 
   Referring to  FIG. 4 , there is shown a PCI configuration space for use with distributed computer system  300  or the like, in accordance with an embodiment of the invention. As is well known, each switch, bridge and adapter in a system such as data processing system  300  is identified by a Business/Device/Function (BDF) number. The configuration space is provided with a PCI configuration header  400 , for each BDF number, and is further provided with an extended capabilities area  402 . Respective information fields that may be included in extended capabilities area  402  are shown in  FIG. 4 , at  402   a . These include, for example, capability ID, capability version number and capability data. In addition, new capabilities may be added to the extended capabilities  402 . PCI-Express generally uses a capabilities pointer  404  in the PCI configuration header  400  to point to new capabilities. PCI-Express starts its extended capabilities  402  at a fixed address in the PCI configuration header  400 . 
   In accordance with the invention, it has been recognized that the extended capabilities area  402  can be used to determine whether or not a PCI component is a multi-root aware PCI component. More particularly, the PCI-Express capabilities  402  is provided with a multi-root aware bit  403 . If the extended capabilities area  402  has a multi-root aware bit  403  set for a PCI component, then the PCI component will support the multi-root PCI configuration as described herein. Moreover,  FIG. 4  shows the extended capabilities area  402  provided with a PCI Configuration Manager (PCM) identification (ID) field  405 . If a PCI component supports the multi-root PCI configuration mechanism, then it will also support PCM ID field  405 . 
   It is to be understood that the PCM ID is a value that uniquely identifies the PCM, throughout a distributed computer system such as system  100  or  300 . More particularly, the PCM ID clearly indicates the root node or CPU set in which the PCM component is located. 
   Referring to  FIG. 5 , there is shown an information space  502 , one of which corresponds to each root node or host CPU set. Each information space  502  includes a number of information fields such as fields  504 - 508 , which provide the vital product data (VPD) ID, the user ID and the user priority, respectively, for its corresponding root node or host CPU set. It is to be understood that other information fields not shown could also be included in each information space  502 . User ID and user priority may be assigned to respective root nodes by a system user, administrator or administration agent. 
   As is known by those of skill in the art, a unique VPD ID is assigned to a host CPU set when the unit is manufactured. Thus, respective host CPU sets of system  300  will have VPD ID values that are different from one another. It follows that to provide a unique value for PCM ID, the host CPU set having the highest VPD ID value could initially be selected to contain the PCM, and the PCM ID would be set to such highest VPD ID value. Alternatively, the host CPU set having the highest user ID, the highest user priority, or the highest value of a parameter not shown in information space  502  could be initially selected to contain the PCM component, and the PCM ID would be such highest value. The root node or host CPU unit initially designated to contain the PCM, and to thereby be the master root node for the system, could be selected by a system user, or could alternatively be selected automatically by a program. 
   Referring further to  FIG. 5 , there is shown information space  502  having an active/interactive (A/I) field  510 . The root node at which the PCM is located shows an active status in its field  510 , and the remaining root nodes of the system each show an inactive status. As an example, host CPU set  332  of system  300  would have an active status in field  510 , since it contains PCM  370 , and host sets  334  and  336  would each have an inactive status. 
   An important function of the PCM  370 , after respective routings have been configured, is to determine the state of each switch in the distributed processing system  300 . This is usefully accomplished by operating the PCM to query the PCI configuration space, described in  FIG. 4 , that pertains to each component of the system  300 . This operation is carried out to provide system configuration information, while each of the other host sets remains inactive or quiescent. The configuration information indicates the interconnections of respective ports of the system to one another, and can thus be used to show the data paths, or routings, through the PCI switches of switched-fabric  144 . 
   Referring to  FIG. 6 , there is shown a fabric table  602 , which is constructed by the PCM as it acquires configuration information. The configuration information is usefully acquired by querying portions of the PCI-E configuration space respectively attached to a succession of active ports (AP), as described hereinafter in connection with  FIG. 7 . 
   Referring further to  FIG. 6 , there is shown fabric table  602  including an information space  604  that shows the state of a particular switch in distributed system  300 . Information space  604  includes a field  606 , containing the identity of the current PCM, and a field  608  that indicates the total number of ports the switch has. For each port, field  610  indicates whether the port is active or inactive, and field  612  indicates whether a tree associated with the port has been initialized. Field  614  shows whether the port is connected to a root complex (RC), to a bridge or switch (S) or to an end point (EP). 
     FIG. 6  further shows fabric table  602  including additional information spaces  616  and  618 , which respectively pertain to other switches or PCI components. While not shown, fabric table  602  in its entirety includes an information space similar to space  604  for each component of system  300 . Fabric table  602  can be implemented as one table containing an information space for all the switches and PCI components in the fabric, or as a linked list of tables, where each table contains the information space for a single PCI switch or PCI component. 
   In systems such as those of  FIGS. 1 and 3 , multiple fabric managers are allowed, and can plug into any part of a multi-root aware switch such as switches  302  and  304 . As a result, and as discussed above, when the current PCM is acquiring PCM identity information from the field  606  of a particular switch, it may happen that a PCM ID associated with the switch is different from the identity of the current PCM. In order to construct a fabric table, the invention provides a challenge protocol to deal with events of this type. 
   Referring to  FIG. 7 , there is shown a procedure usefully carried out by the PCM, in order to construct the fabric table  602 . Generally, the PCM successively queries the PCI configuration space of each switch and other PCI component. This is done to determine the number of ports a component has and whether respective ports are active ports (AP) or inactive ports. The PCM then records this information in the fabric table, together with the VPD ID of the PCI component. 
   Function block  702  and decision block  704  indicate that the procedure of  FIG. 7  begins by querying the configuration space to find out if the component attached to a port AP is a switch. Function block  706  shows that if the component is a switch, the field “Component attached to port (AP) is a switch” is set in the PCM fabric table. Then, in accordance with decision block  707 , it becomes necessary to determine whether the switch being queried already shows a PCM ID, either the identity of the currently active PCM or a different PCM. More specifically, decision block  707  requires determining whether field  606  of the switch does or does not show a PCM ID that is equal to 0. 
     FIG. 7  further shows that if the determination of decision block  707  is positive, the ID of the current PCM, which is engaged in constructing the fabric table, is set in the PCM configuration table of the switch, in accordance with function block  708 . This table is the information space in fabric table  602  that pertains to the switch. Function block  710  shows that the fabric below the switch is then discovered, by re-entering this algorithm for the switch below the switch of port AP in the configuration. Function block  712  discloses that the port AP is then set to port AP-1, the next following port, and the step indicated by function block  702  is repeated. 
   Referring again to decision block  707  of  FIG. 7 , it is seen that if the determination of block  707  is negative, the switch being queried must contain a non-zero value for PCM ID. Accordingly, as shown by decision block  730 , it becomes necessary to determine whether or not this PCM ID value is equal to the current PCM ID, that is, the PCM in control of the system. If such determination is positive, function block  732  indicates that the PCM disables the port connection to the switch, and records in the fabric table that a loop was found. The task set forth at function block  720 , which is described hereinafter in further detail, is then carried out. 
   Referring further to decision block  730  of  FIG. 7 , a negative result for the query thereof would indicate that the switch had a PCM ID that was different from the current PCM ID. In this event, it becomes necessary to invoke the PCM challenge protocol, as shown by function block  734 . This protocol is described hereinafter, in connection with  FIG. 8 , and either will or will not be won by the current PCM ID, in accordance with decision block  736 . If the challenge is won, the procedure of  FIG. 7  is again advanced to function block  720 . If the challenge is lost, the procedure is aborted, as shown by function block  738 . 
   Referring further to decision block  704  of  FIG. 7 , if the component being queried is not a switch, it becomes necessary to determine if the component is a root complex or not, as shown by decision block  714 . If this query is positive, the message “Component attached to port AP is an RC” is set in the PCM fabric table, as shown by function block  716 . Otherwise, the message “Component attached to port AP is an end point” is set in the PCM fabric table, as shown by function block  718 . In either event, the port AP is thereupon set to AP-1, as shown by function block  720 . It then becomes necessary to determine if the new port AP value is greater than zero, in accordance with decision block  722 . If it is, the step of function block  702  is repeated for the new port AP. If not, the process of  FIG. 7  is brought to an end. 
   When the fabric table  602  is completed, the PCM writes the configured routing information that pertains to a given one of the host CPU sets into the root complex of the given host set. This enables the given host set to access each PCI adapter assigned to it by the PCM, as indicated by the received routing information. However, the given host set does not receive configured routing information for any of the other host CPU sets. Accordingly, the given host is enabled to access only the PCI adapters assigned to it by the PCM. 
   Usefully, the configured routing information written into the root complex of a given host comprises a subset of a tree representing the physical components of distributed computing system  300 . The subset indicates only the PCI switches, adapters and bridges that can be accessed by the given host CPU set. 
   As a further feature, only the host CPU set containing the PCM is able to issue write operations, or writes. The remaining host CPU sets are respectively modified, to either prevent them from issuing writes entirely, or requiring them to use the PCM host set as a proxy for writes. 
   Referring to  FIG. 8 , there is shown a flow chart depicting a challenge protocol for an embodiment of the invention. Function block  802  indicates that the protocol is entered when a PCI switch is found to show a PCM ID that is not the current PCM ID, as described above in connection with function block  734  of  FIG. 7 . Upon entering the protocol, a message challenging the switch configuration is sent to the root node identified by the PCM found at the switch, referred to hereinafter as the challenge PCM. As shown by function block  804 , the challenge message is directed to the BDF number of the identified root node. After the message is sent, function block  806  indicates that a timer loop (TL) is set to an integer N associated with a time period. N could, for example, be 5 and the time period could be 5 milliseconds. Function block  806  shows that a corresponding cycle time X is also selected. If the cycle time was selected to be 1 millisecond, 5 cycles or iterations would occur until the time period associated with N came to an end. As described hereinafter, the values pertaining to function blocks  806  and  808  are respectively selected to establish a maximum period for response. 
   Referring further to  FIG. 8 , decision blocks  810  and  812  indicate that the challenge PCM may respond to the challenge message, sent to the identified root node, by providing its challenge PCM ID. If such response is received by the current PCM, the challenge PCM ID is compared with the current PCM ID, as shown by decision block  812 . If the current PCM ID is found to be greater than the challenge PCM ID, confirmation is provided that the current PCM is indeed the correct PCM. Accordingly, the challenge is recorded to be won, as shown by function block  814 , and the protocol is exited at  818 . Thereupon, the procedure shown in  FIG. 7  is advanced to function block  720  thereof. 
   In the event that the challenge PCM ID is found to be equal to or greater than the current PCM ID, the challenge will be recorded as being lost, as indicated by function block  816 . The protocol will be exited and the procedure of  FIG. 7  will be aborted, in accordance with function block  738 . 
   Referring further to decision block  810  of  FIG. 8 , if the challenge PCM does not respond to the challenge message within the cycle time, the timer loop TL is decremented by one, as shown by function block  820 . For a TL of 5, TL- 1  would go to 4. If TL was not 0, the protocol would return to function block  808 , in accordance with decision block  822 . The protocol would then wait for another period of cycle time X for the challenge PCM to respond to the message. After a number of such iterations with no response, TL will reach 0. When this occurs, an error message is created and the configuration pertaining to the switch is aborted, as shown by function block  824 . 
   A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. 
   The computer program code may be accessible from a computer-usable or computer-readable storage medium for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any tangible apparatus that can contain and store the program for use by or in connection with the instruction execution system, apparatus, or device. 
   The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or a semiconductor system. The medium also may be physical medium or tangible medium on which computer readable program code can be stored. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk, an optical disk, or some other physical storage device configured to hold computer readable program code. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
   Input/output or I/O devices (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. 
   Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.