Abstract:
A simulation technique that handles accesses to a frame of memory via a proxy object provides improved throughput in simulation environments. The proxy object, if present, processes the access at a head of a linked list of frames. If a check frame is not inserted in the list, the memory frame handles the request directly, but if a check frame is inserted, then the check operation is performed. The check frame can be a synchronization frame that blocks access to a memory frame while the check frame is present, or the check frame may be a breakpoint, watch or exception frame that calls a suitable handling routine. Additional check frames may be chained between the interface and the memory subsystem to handle synchronization, breakpoints, memory watches or other accesses to or information gathering associated with the memory frame.

Description:
[0001]    The present application is a Continuation of U.S. patent application Ser. No. 13/707,812, filed on Dec. 7, 2012 and claims priority thereto under 35 U.S.C. 120. The disclosure of the above-referenced parent U.S. patent application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     Description of Related Art 
       [0002]    Simulators, debuggers and accelerators that can be implemented in hardware, software or a combination of both, are typically are used in verification of program code as well as verification of designs of processor integrated circuits (ICs) and other large-scale logic. 
         [0003]    In simulation using a multi-threaded simulator, the handling of memory accesses is complicated, as hardware mechanisms that are provided to avoid memory access conflicts are not actually present in the simulator, and thus the simulator must handle memory accesses made during the simulation in a synchronized manner, which reduces simulation throughput. Further, debug facilities such as breakpoints, watches and protection exceptions are often set for memory locations or memory ranges, requiring the simulator to perform those checks in the simulation environment. 
         [0004]    Therefore, it would be desirable to provide a mechanism for managing accesses to memory that improves simulation throughput while synchronizing accesses and providing breakpoint, watch and protection capability for the simulator. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The invention is embodied in a computer-performed simulation method that simulates operation of a computer system and manages accesses to memory frames to perform the simulation. 
         [0006]    The simulation method manages accesses to a frame of memory by using proxy frames to receive accesses to memory frames of a memory subsystem. Check frames can be inserted between the proxy frame and the memory frames, which if present, pre-process the accesses to the memory frames. If a check frame is not present for the accessed frame, the memory frame handles the request received from the proxy frame. The check frames can track write accesses to memory so that dirty frames can be tracked. The check frame may be a synchronization frame that stalls accesses from other simulator threads in a multi-threaded simulation environment. Other check frames may be chained between the proxy object and the memory frame to handle memory synchronization, breakpoints, memory watches, protections or for tracking accesses to the memory for other applications. 
         [0007]    The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0008]    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 the invention when read in conjunction with the accompanying Figures, wherein like reference numerals indicate like components, and: 
           [0009]      FIG. 1  is a pictorial diagram depicting a simulation architecture in which the techniques disclosed herein can be practiced in accordance with embodiments of the invention. 
           [0010]      FIG. 2A  and  FIG. 2B  are pictorial diagrams of simulated or actual memory access paths in the simulation architecture of  FIG. 1A  or the computer architecture of  FIG. 1B . 
           [0011]      FIG. 3  is a pictorial diagram depicting on example of check frame  16 A of  FIG. 2B . 
           [0012]      FIGS. 4A-4C  are pictorial diagrams depicting a sequence of steps in a method of synchronization in accordance with an embodiment of the present invention. 
           [0013]      FIGS. 5A-5C  are pictorial diagrams depicting a sequence of steps in a method of synchronization in accordance with another embodiment of the present invention. 
           [0014]      FIG. 6  is a flowchart depicting a method of managing memory access in accordance with an embodiment of the present invention. 
           [0015]      FIG. 7  is a block diagram illustrating a computer system in which program code according to an embodiment of the present invention implementing a method according to an embodiment of the invention is executed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The present invention encompasses computer-performed simulation methods for managing accesses to memory in simulated processing systems. Accesses to memory frames are made from corresponding proxy frames, which are objects having an interface for handling the accesses to the memory frames. A check frame may be inserted between the proxy frame and the memory interface and the simulated memory subsystem that contains the memory location. For the purposes of illustration, it can be assumed the frame size and the page size of the system being simulated are the same, but different frame sizes can be used in an actual implementation. 
         [0017]    The proxy frame is generally allocated when the memory frame is referenced and de-allocated when all of the references to the memory frame no longer exist. Multiple check frames may be inserted for a given frame, the arrangement of which is maintained via the linked list. Functions such as synchronization between simulator and/or simulated threads, breakpoints, memory watches and dirty page management can be implemented and easily inserted and removed by inserting independent check frames corresponding to those functions. 
         [0018]    Referring now to  FIG. 1 , a simulation architecture is shown, in which techniques according to embodiments of the invention can be practiced. A simulator  10 , i.e., a simulation program, implements a memory interface  12 , through which a simulated memory subsystem  14  is accessed. Simulated memory subsystem  14  is generally data memory controlled by simulator  10  that contains stored values that represent the contents of memory that is being simulated along with one or more simulated processors that access the memory. The simulated memory may be data and/or code memory. Memory interface  12  is illustrated as a read( ) and write( ) interface that corresponds to load and store instructions, respectively, that are executed by the simulated processor. In the present invention, as will be illustrated in further detail below, for memory frames that require a check, a proxy frame handles the requests from memory interface  12  at simulated memory subsystem  14 , so that synchronization between multiple simulation threads, breakpoints, watches, protection and other mechanisms that require special handling can be implemented for particular frames of memory. Each frame of memory simulated by simulated memory subsystem  14  that has a check set or one or more memory locations has an associated proxy frame and may have one or more check frames chained between the proxy frame and the memory frame. Memory frames that do not have an associated check are referenced directly. The frames of memory generally correspond to memory pages in the simulated system, but that may be alternatively larger or smaller than a page. The per-frame application of proxy frames and insertion of check frames provides direct access for frames that have no special handling, so that throughput is maximized, because no special checks are required to determine if special handling is required for that memory frame. 
         [0019]    Referring now to  FIG. 2A , a simulated memory access path for a memory frame within the simulation environment of  FIG. 1  is shown. In the Figure, simulator  10  is illustrated as accessing frame proxy  16  through memory interface  12 . Frame proxy  16  receives requests for access to a particular memory frame  18  of simulated memory subsystem  14  and handles the access to memory frame  18 .  FIG. 2A  shows the access path when no checks are present for any location within memory frame  18 .  FIG. 2B  shows an access path for memory frame  18  for which a check frame  18 A has been inserted between frame proxy  16  and memory frame  18 . Frame proxy  16  receives access requests from simulator  10  (or alternatively from processor  51  through memory access subsystem  50  as in  FIG. 1B ), which passes the access on to check frame  18 A, which performs special handling for the frame, e.g., simulator synchronization, simulated thread synchronization, breakpoint, watch or exception handling, or other special processing. Check frame  18 A then passes the access request to the next frame in the linked list, which in the depicted example is memory frame  18 . Any number of check frames  18 A can be inserted to perform special processing, since each frame contains a pointer to the next frame in the list, ending with memory frame  18 . In each of the cases shown in  FIG. 2A  and  FIG. 2B , access to simulated memory subsystem  14  may or may not be required, since one of the inserted proxy frames may prevent access to the requested memory location, depending on what type of special operations are required by the check frames ahead of memory frame  18  in the linked list formed by the frames. 
         [0020]    Referring now to  FIG. 3 , an architecture of check frame  18 A of  FIG. 2B  is shown in accordance with an embodiment of the present invention. Check frame  18 A contains a table  20  with at least one special handling entry of the form as given below in Table I: 
         [0000]                                                  TABLE I                       Location   Type                                            Reservation Frame                   Trap Frame   &amp;Spinlock           Address1   Breakpoint   &amp;breakpoint_handler           Address 2   Watch   &amp;watch_handler           Address 3   Exception   &amp;exception_handler               Mark Dirty Frame               Zero Frame (or init value   (init_value)               frame)               Swap Frame   &amp;Spinlock                        
The types of check frames illustrated in Table I are generally individual frames placed in the chain of linked frames for each given check, but alternatively, check frames may contain multiple checks, for example, a single breakpoint, watch or exception frame could be used to store breakpoint, watch or exception entries for multiple storage locations. The reservation frame is a frame placed in a chain to indicate that a thread being simulated has placed a reservation on one or more memory locations within the memory frame, e.g., a cache line. If another simulated thread attempts to write to a location within the reserved memory, the reservation is lost, providing support for simulating atomic operations, such as those implemented by POWER PC processors (POWER PC is a trademark of International Business Machines Corporation). A trap frame is a frame placed in front of the memory frame during simulation while a simulation thread is accessing the memory frame. If another simulation thread attempts to access the memory frame, the trap frame automatically receives the access. Further details of the use of reservation and trap frames will be illustrated below with reference to  FIGS. 4A-4C . Breakpoint, watch and exception check frames call corresponding handlers, or trigger corresponding events, when a location (or alternatively a range of memory locations) in the frame are accessed in a request. A mark dirty frame can be inserted on a first write to a location within a frame, so that checks on the dirty bit and setting of the dirty bit do not have to be performed for each subsequent memory write to locations within the frame. A zero frame (or initial value frame) indicates that the memory frame has been initialized with zero (or init_value), and thus zero (or init_value) can be returned on a read access, without requiring access to the underlying memory frame. A swap frame is used in a special form of the synchronization implemented with the reservation frame, and will be discussed in further detail below with reference to  FIGS. 5A-5C . Check frame  18 A further contains a next object pointer  22  The value stored in next object pointer  22  can be placed in an upstream object&#39;s next object pointer when check frame  18 A is removed from the chain of check frames, since the chain of check frames forms a linked list with next object pointer  22  providing the links. The above is only an example of a data structure that can be used within proxy frame to represent special handling operations and other forms of information that can provide some or all of the above-described capabilities. The data structure described above can alternatively be implemented by check frame  18 A.
 
         [0021]    Referring now to  FIGS. 4A-4C , a synchronization process in accordance with an embodiment of the present invention is shown.  FIGS. 4A-4C  illustrate a progression through time as various events in potentially conflicting accesses by two simulator threads HT 1  and HT 2  are received at the same frame proxy  26 A. In  FIG. 4A , frame proxy  26 A receives a reservation request for a thread simulated by simulator thread HT 1 . The request causes a reservation frame  26 B to be created. Reservation frame is inserted after frame proxy  26 A and is maintained unless another simulated thread writes to the corresponding memory frame, which will cause a loss of the reservation and reservation frame  26 B will be removed. If another simulator thread HT 2  attempts to simulate an access to the memory frame while reserved frame is being accessed by the simulated thread that placed the reservation, frame proxy  26 A sends the request to a trap frame  26 C instead of sending the request onward to the underlying memory frame. Frame proxy  26 A accomplishes this diversion by setting the head of the frame chain to point to trap frame  26 C. Simulator thread HT 2 , which has entered trap frame  26 C, waits on a spinlock  28  that has been initialized by frame proxy  26 A. In the wait loop for spinlock  28 , trap frame  26 C compares the pointer at the head of the frame proxy  26 A to its own pointer. As illustrated by  FIG. 4C , Spinlock  28  is released once frame proxy  26 A receives the response to the access by the simulated thread that placed the reservation. In response to completing the access, frame proxy  26 A removes trap frame  26 C from the head of the frame chain, which is, in turn, detected by the wait loop for spinlock  28  as described above. Frame proxy  26 A sets the head of the frame chain to point again to reservation frame  26 B which remains in place until the reservation is lost or otherwise removed. 
         [0022]    Referring now to  FIGS. 5A-5C , a synchronization process in accordance with another embodiment of the present invention is shown.  FIGS. 5A-5C  illustrate a progression through time as various events in potentially conflicting accesses by two host threads HT 1  and HT 2  are received at the same frame proxy  26 A. The depicted process reduces locking/unlocking overhead that occurs, for example, when a single thread repeatedly accesses a memory frame chain that implements the process illustrated in  FIGS. 4A-4C . As in  FIG. 4A , in  FIG. 5A , frame proxy  26 A receives a reservation request in response to a simulation being carried out by simulator thread HT 1 . The reservation request causes a reservation frame  26 B to be created and inserted in the frame chain. Frame proxy  26 A also creates a swap frame  26 D and inserts swap frame  26 D at the head of the linked-list forming the frame chain, to prevent other simulator threads, e.g., simulator thread HT 2 , from simulating access to the same memory frame while program code simulated by simulator thread HT 1  is still accessing the memory frame. The presence of swap frame  26 D indicates to other threads that thread HT 1  is holding a lock on an access to the memory frame and might not be able to release the memory frame. At the end of each access to the memory frame, thread HT 1  retains the pointer to the head of the actual frame chain, i.e., a pointer to reservation frame  26 B, so that subsequent accesses can be performed directly through the frame chain beginning at reservation frame  26 B. Then, as illustrated in  FIG. 5B , eventually program code simulated by simulator thread HT 2  attempts to access the memory frame via frame proxy  26 A. The access is passed to swap frame  26 D, which waits on a spinlock  28  that remains locked until release of the frame chain by the thread(s) simulated by simulator thread HT 1 . At the same time, simulator thread HT 1  is sent a request to release the frame chain. As with the trap frame spinlock in  FIGS. 4A-4C , swap frame  26 D compares the pointer at the head of the frame chain to itself to determine whether simulator thread HT 1  has caused frame proxy  26 A to release the frame chain. Next, as shown in  FIG. 5C , once the frame chain is released, frame proxy  26 A provides the pointer to reservation frame  26 B to thread HT 2  and also maintains swap frame  26 D at the head of the frame chain, so that another simulator thread cannot obtain access to the memory frame. The synchronization process illustrated in  FIGS. 5A-5C  is an alternative to that of  FIGS. 4A-4C  and a system could determine which process to use via metrics, for example, measuring the number of sequential simulated accesses by a given simulator thread versus simulated accesses by alternating simulator threads. 
         [0023]    Referring now to  FIG. 6 , a method of simulating accesses in accordance with an embodiment of the invention is exemplified. The simulation is started (step  30 ) and when a memory access is received (step  31 ), if a check frame is not inserted ahead of the corresponding memory frame (decision  32 ), then the value is accessed in memory via the memory frame (step  33 ). If there was no check frame inserted ahead of the memory frame (decision  32 ), until the simulation is complete (decision  41 ), steps  31 - 38  are repeated for subsequent memory accesses. If a check frame was inserted for the accessed frame (decision  32 ), then if the check frame contains a breakpoint, watch or protection entry (decision  34 ), the handler is called or triggered (step  35 ). If the check frame is a trap frame (decision  36 ), the access is stalled until the trap frame is removed (step  37 ). Until the last check frame in the chain (linked list) is encountered (decision  38 ), steps  34 - 38  are repeated, and steps  31 - 39  are repeated until the simulation is complete. While the zero (or init_value) check frame handling is not shown in  FIG. 6 , a zero frame should generally be inserted last in the chain before the memory frame itself and accessed instead of the memory frame at step  33 , so that other checks can be performed ahead of returning the zero (or initial) value. 
         [0024]    Referring now to  FIG. 7 , a computer system in which the above-described computer-performed methods are performed, is shown. A general-purpose computer  100  is coupled to a graphical display  102  for display of simulation results, as well as user interface screens for controlling one or more programs. The programs include programs forming computer program products including program instructions for carrying out the methods described above. Input devices such as a keyboard  108 A and a mouse  108 B provide for user interaction with general-purpose computer  100 . General-purpose computer  100  includes a processor  104  for executing program instructions stored in a memory  106  including program instructions forming the above-described computer program products in accordance with embodiments of the invention. A removable storage device, such as a DVD-ROM drive  105  is included for accepting storage media such as DVD-ROM DVD 1  forming a computer program product in accordance with an embodiment of the invention. The computer system depicted in  FIG. 7  is only one example of a computer system, such as a desktop computer system, or a notebook computer system, that can be used to perform the simulations described above. Other types of computer systems, including distributed and networked computer systems in which some or all of the algorithms and portions thereof are executed remotely, are within the scope of the invention as contemplated herein. The system depicted in  FIG. 7  should not be construed as limiting the type of computer system or computer program products that form embodiments of the present invention. 
         [0025]    As noted above, portions of the present invention may be embodied in a computer program product, which may include firmware, an image in system memory or another memory/cache, or stored on a fixed or re-writable media such as an optical disc having computer-readable code stored thereon. Any combination of one or more computer-readable medium(s) may store a program in accordance with an embodiment of the invention. The computer-readable medium may be a computer-readable signal medium or a computer-readable storage medium. A computer-readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer-readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. 
         [0026]    In the context of the present application, a computer-readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. A computer-readable signal medium may include a propagated data signal with computer-readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer-readable signal medium may be any computer-readable medium that is not a computer-readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. Program code embodied on a computer-readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
         [0027]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.