Abstract:
The present invention relates generally to efficient message passing support and memory access protections in scalable shared memory multiprocessing computer systems. In a multiprocessor system, processors need to communicate with one another to coordinate their work. Prior art multiprocessors only permit message passing or unprotected direct memory access. The present invention allows direct memory access with protection. The mechanism of the present invention permits processors to directly access each other&#39;s memory while retaining protection against faulty software or hardware. Security in the face of malicious intent of the communicating software is not maintained in the preferred embodiment, although a variation of the mechanism provides additional protection against malicious software albeit at the expense of slightly more complex hardware.

Description:
CROSS-REFERENCE TO CO-PENDING APPLICATION 
     This application is related to co-pending application Ser. No. 09/003,721, filed Jan. 7, 1998, entitled, “Cache Coherence Unit With Integrated Message Passing and Memory Protection for a Distributed, Shared Memory Multiprocessor System,” by inventor Wolf-Dietrich Weber, to common assignee. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to computer communication protocols, and more specifically to a message passing protocol which is integrated with a cache coherence protocol in a multiprocessing computer system. 
     2. Discussion of Background Art 
     Multiprocessor data computer systems consist of a plurality of processor nodes communicating over a high-speed interconnection network. Each processor node typically includes a processor and local Random Access Memory (RAM). A computational problem is divided among processor nodes so that the utilization of particular resources available at different processor nodes is maximized. Dividing the problem among processor nodes also reduces the time needed to produce a result and thereby expedites the computation. However, this division of labor necessarily implies that a process running on one processor node may depend on the results of computations being performed at another processor node. The various processes then must communicate over the interconnection network to exchange information relevant to their particular problems, and must also synchronize the processes. 
     The performance level of a multiprocessor system depends on the speed with which processors can communicate with one another. In the sharedmemory paradigm, communication is very fast because each processor can simply read what the other processors have written. However, this model does not offer communicating processes any protection from interfering with one another by inadvertently overwriting each other&#39;s critical memory areas. In the message-passing model, on the other hand, each processor can only access its own memory and can only communicate with other processors by explicitly building up a message and sending it to the other processor. This model offers the communicating processors protection from one another, because they cannot write to each other&#39;s memory. However, this model is also inefficient because typically the operating system must be invoked on both sides of the transfer (sender and receiver). These operating system calls slow communication between the processors. It is thus desirable to allow communicating processes access to designated areas in each other&#39;s memory directly but at the same time protecting against inadvertent accesses to other areas of memory, all without the need for operating system intervention on the receiving side. 
     A mechanism that allows one processor to protect itself from having another processor corrupt its memory inadvertently due to a hardware or software fault is disclosed in U.S. Pat. No. 5,448,698, issued Sep. 5, 1995 to Wilkes. The Wilkes protection-check mechanism uses a protection table at the target that specifies a key for a number of memory areas. The mechanism disclosed in the Wilkes patent has the disadvantage that a table must be built into the hardware and requires storage space that inherently contains only a limited number of entries. 
     SUMMARY OF THE INVENTION 
     The present invention resides in a computer system having a plurality of processor nodes and an interconnection. Each processor node is connected to the interconnection and has a memory and a memory bus connected to the memory. Each processor node has a plurality of processors and each processor has a cache. The computer system has a mesh coherence unit for controlling messages and memory access requests between the memory bus and the interconnection. The computer system has a lock and key mechanism wherein a processor node having its memory accessed retains a lock value to compute, by a function from a memory address, a key value and a temp value, and a processor node requesting a memory access is permitted memory access when the temp value matches the key value. 
     One object of the present invention is to provide a message passing system where operating system calls are not required on the target side. 
     Another object is to provide protection for processes running at different processor nodes to protect each process from damage. 
     Still another object is to avoid the use of a table that requires additional hardware and storage space, and which inherently can contain only a limited number of tabular entries. 
    
    
     These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of a computer system of the present invention having multiple processor nodes; and 
     FIG. 2 is a functional block diagram of a processor node of FIG. 1; 
     FIG.  3 A and FIG. 3B show one embodiment for setting up a lock; 
     FIG.  4 A and FIG. 4B show one embodiment for granting access rights to page x for an initiator node by the target node making key x; and 
     FIG. 5A and 5B show one embodiment of how access from a remote node can be made between a target and initiator node. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention resides in a system and method for a communication protocol in scalable shared-memory multiprocessing computer systems. A scalable shared-memory multiprocessing computer system has a plurality of processors connected to an interconnection over which the processors communicate with each other. The conventional mechanism by which a message is passed is via an input/output channel and an interconnection. 
     In a system with the present invention, the same communication channel is used for both cache coherence and message passing, which significantly increases the rate at which messages are exchanged. Each processor communicates with processors in other processor nodes by sending and receiving messages using a message-passing protocol that is tightly integrated with an inter-processor node cache coherence protocol. At the same time, a lock and key mechanism raises protection barriers between processor nodes that communicate only via message passing. In contrast to the prior art Wilkes device, supra, the present invention requires very little hardware storage and can cover an unlimited number of memory areas. 
     FIG. 1 is a functional block diagram of a computer system  100  according to the invention, including multiple processor nodes  102   a-t  and a processor node interconnection  104  which provides point-to-point communication between the nodes  102   a-t . Each of the processor nodes  102   a-t  may be configured as a stand-alone computer system or associated with other processor nodes to share memory. The term “site” is used to designate a group of processor nodes sharing a physical address space in memory. Selected processor nodes  102   a-d ,  102   f-i ,  102   l-o , and  102   q-t  are respectively configured as sites  106   a ,  106   b ,  106   c  and  106   d . Other processor nodes  102   e ,  102   j ,  102   k , and  102   p  are also connected via interconnection  104 , but do not share memory and thus are not common sites. Such processor nodes in different sites communicate via message passing. For example, processor nodes in site  106   a  communicate with processor nodes in other sites, e.g., processor node  102   n  in site  106   c , by sending messages via interconnection  104 . 
     The present invention achieves cache coherence with integrated message passing between processor nodes  102   a-t , as shown in exemplary system  100  (FIG.  1 ). The processor nodes in a site, e.g., processor nodes  102   a-d  in site  106   a , share a physical address memory space. In addition, each of the processor nodes has multiple processors  202   a-d  with a respective cache memory  204   a-d  (FIG.  2 ). Thus, cache coherence needs be maintained between caches  204   a-d  of processors  202   a-d  not only within a node  102 , but also in different processor nodes  102   a-d . For example, caches in node  102   a  must be coherent with caches in nodes  102   b-d.    
     The invention further includes a memory protection mechanism. The memory protection mechanism permits access to a shared physical address space for processor nodes within a site  106   a-d  and denies access to the shared physical address space for processor nodes  102   a-t  outside the site  106   a-d . For example, processor node  102   e  may pass messages to processor node  102   a . However, because processor node  102   e  is not within site  106   a , processor node  102   e  may not perform memory access operations on the physical address space of site  106   a.    
     FIG. 2 is a functional block diagram of a processor node  102 . Processor node  102  is exemplary of FIG. 1 processor nodes  102   a-t  and includes processors  202   a-d  each having a respective cache  204   a-d , a memory subsystem  206 , an input/output subsystem  208 , and a mesh coherence unit (MCU)  210 . Each of the functional units  202   a-d ,  206 ,  208 , and  210  are connected to bus  212  for transmitting control, address, and data signals between the units. The mesh coherence unit  210  is connected to interconnection  104 . 
     Processors  202   a-d , memory subsystem  206 , input/output subsystem  208 , and bus  212  are commercially available, with one or more processors per node. The mesh coherence unit  210  coordinates inter-processor node cache coherence, inter-processor node message passing, and inter-processor node memory protection. 
     In a multiprocessor system, different processors typically must communicate with one another to co-ordinate their work. In order to limit the possibility of faulty software or hardware of one processor from corrupting another processor, and/or in order to enforce access security between different processors, some multiprocessors do not permit one processor to read directly from or to write directly to the memory of another processor. Instead, these multiprocessors only allow processors to exchange messages. Unlike a direct memory access, a message has to be processed and screened by the receiving processor, and hence this type of communication is typically less efficient. 
     The mechanism of the present invention permits processors to access each other&#39;s memory directly while permitting processors to retain protection against faulty software or hardware. However, this mechanism does not offer security against malicious intent of the communicating software. In another embodiment, the mechanism provides protection against malicious software, but requires slightly more complex hardware. 
     The mechanism relies on a lock and key scenario. The processor node that is performing an access (the “initiator” node) uses a key, while the processor node being accessed (the “target” node) keeps a lock. Each target generates a large number serving as the lock. This number is stored in hardware that does not accept memory access requests from other processor nodes. 
     FIG.  3 A and FIG. 3B show one embodiment for setting up the lock. FIG. 3A is a block diagram of the software step  310  to generate the lock and the hardware step  320  for storing the lock in access protection hardware. FIG. 3B shows that the lock  350  resides within the protection check mechanism  330  and that the protection check mechanism  330  resides within the mesh coherence unit (MCU)  340 . 
     Access protection is maintained on a per memory page basis, where a page typically represents about 4 kilobytes of memory. If a target wishes to grant access rights of a particular page to some initiator, it manufactures a key by using the equation: 
     
       
         key=f(lock, addr) 
       
     
     where lock is the lock number, addr is the address of the page for which the key is manufactured, and f is a simple function. The key and address are then passed to the initiator. 
     FIG.  4 A and FIG. 4B show one embodiment for granting access rights to page x for an initiator node by the target node making key x. FIG. 4A is a block diagram showing the software step  410  to manufacture the key from the lock and address and the software step  420  for sending the key to the node that is being granted access rights to page x. FIG. 4B shows how the key  430  is passed through the interconnect  450  between the mesh coherence units (MCU)  440  and  460 . 
     When the initiator wishes to access the page, it sends the key along with the address to the target. At the target, hardware checks the access by computing a “temp” value from the incoming address and lock: 
     
       
         temp=f(lock, addr) 
       
     
     If the temp value matches the key passed with the access, then the access is allowed to proceed. Otherwise, the access is rejected. 
     FIG. 5A and 5B show one embodiment of how access from a remote node can be made between a target and initiator node. FIG. 5A is a block diagram showing the access request to page x arrives with the key in step  510 . The temp value is computed as a function of the lock and address in step  520 . The temp value is compared to the key value in step  530  and if they are equal then the access request is accepted in step  540 , otherwise the access request is rejected in step  550 . FIG. 5B shows how the access request  590  is accepted or rejected through the interconnect  570  between the mesh coherence units (MCU)  560  and  580 . 
     In its simplest form, function f is simply an EXCLUSIVE-OR operation. The mechanism then offers protection against accidental access to an area of memory to which access has not been granted as long as the software of the initiator only uses keys that are given to it by the target. In the case where function f is easily inverted, it is relatively easy for malicious software to generate a key that deduces the target&#39;s lock value: 
     
       
         lock=f2(key, addr) 
       
     
     Where f2 is the inverse of f, such that: 
     
       
         x=f2(f(x,y),y) 
       
     
     Thus, there exists the potential for malicious software to manufacture keys for other areas of the target&#39;s memory, and in this case, the mechanism does not provide protection. 
     However, by using a function f that is not easily inverted, protection against malicious software also can be achieved. For example, if 
     
       
         key=mod (addr, lock) 
       
     
     and addr is chosen to have a sufficient quantity of bits, then it becomes very time-consuming for malicious software to calculate the lock value from a given address/key combination. Security can be maintained so long as the lock value is changed more frequently than the reverse calculation can be performed by the malicious software. 
     The exemplary embodiments described herein are for purposes of illustration and are not intended to be limiting. Therefore, those skilled in the art will recognize that other embodiments could be practiced without departing from the scope and spirit of the claims set forth below.