Synchronization of load operations using load fence instruction in pre-serialization/post-serialization mode

Memory accesses in a computer system typically follow a given set of rules to ensure that all agents in the system are working with the correct version of the data. The most rigid set of rules, referred to as strong ordering, requires among other things that all loads to a given agent are observable by other agents in the order in which they were issued, i.e., the program order.

While strong ordering guarantees correctness, it does so at the expense of performance. Processors can achieve high performance for loading information from memory if they are allowed to follow weaker rules. The weak ordering rule gives the processor the freedom to choose the highest performance way to load data from memory.

The weak ordering rule allows instructions to be executed out of order. This rule enhances processor's performance because instructions can be executed as soon as resources are available, avoiding wasteful idle periods.

However, at a particular reference time, the program running on a given agent may need to guarantee that all previous loads (or reads) from memory have been observed by all other agents. In addition, the program may also want to ensure that all loads by an agent subsequent to a particular reference time will not be observable before any previous loads. In essence, an agent may want to synchronize all loads issued by itself with respect to a particular timing point.

Prior art synchronization methods include the use of input/output (I/O) instructions, privileged instructions, uncacheable memory references, serializing instructions, and locked instructions. These methods implement the synchronization as part of their primary functions. However, these methods have a number of drawbacks. First, they all require the use of at least one register, taking away valuable storage resources. Second, they are slow due to the time spent to perform the primary function. Third, except serializing and locked instructions, other methods are privileged and not available to the applications users.

Therefore there is a need to provide an efficient method to synchronize load operations using minimal hardware resources.

The features and advantages will become apparent from the following detailed description in which:

FIG. 1 is a diagram illustrating a system according to one embodiment.

FIG. 2 is a diagram illustrating a processor according to one embodiment.

FIG. 3 is a diagram illustrating a relationship and data paths between the reservation unit and the memory control elements according to one embodiment.

FIG. 4 is a diagram illustrating a data flow between different elements in the processor according to one embodiment.

FIG. 5 is a diagram illustrating a control register for mode selection according to one embodiment.

FIG. 6 is a flowchart illustrating synchronizing load operations according to one embodiment.

In the following description, for purposes of explanation, numerous details are set forth. However, it will be apparent to one skilled in the art that these specific details are not necessary. In other instances, well-known electrical structures and circuits are shown in block diagram form.

FIG. 1 is a diagram illustrating a computer system according to one embodiment 100 . While one exemplary computer system is described, various implementations can be used in any number of different computer systems (e.g., one that has more or less cache levels, one that uses a different register renaming and/or out-of-order execution architecture). For simplicity, only relevant elements of the system 100 are shown. The computer system 100 includes the processor 110 , a backside bus 275 , a level 2 (L 2 ) cache subsystem 132 , the processor bus 115 , the memory controller 130 , and the storage device 135 .

The processor 110 includes the decoder 116 , a reservation station 210 , a re-order buffer and register file 220 , the execution unit 112 , a memory cluster 230 , a bus controller 113 , and a write-back data bus 255 . The memory cluster 230 further includes a memory ordering unit 240 and a Level 1 (L 1 ) cache controller 250 (L 1 CC). For simplicity, the L 1 cache memory is not shown. The L 1 cache controller 250 contains the control register 510 having two mode bits MPRE and MPOST.

The decoder 116 issues instructions to the reservation station 210 and to the re-order buffer 220 . The reservation station 210 dispatches the decoded instruction to the execution unit 112 and the memory cluster 230 . If the instruction is ready to be executed, the execution unit will carry out the operation. If the instruction is a memory-referencing instruction, it will be dispatched to the memory ordering unit 240 for preparation for access the L 1 cache (for cacheable instructions only) via the L 1 cache controller 250 .

The write-back bus 255 provides the data path for the L 1 cache controller 250 or the execution unit 112 to return the data back to the re-order buffer and the register file 220 and the reservation station 210 .

The bus controller 113 provides an interface between the processor 110 and the L 2 cache subsystem 132 and the bus 115 . The bus controller 113 includes an external bus controller 262 and a backside bus controller 266 . The external bus controller 262 provides the interface to the bus 115 to which the memory controller 130 is connected. The backside bus controller 266 provides an interface to the L 2 cache subsystem 132 via the backside bus 275 .

Basic Processor Architecture

FIG. 2 illustrates a processor 110 1 including a decode unit 116 , a set of registers 114 , a bus controller 113 , a memory cluster 230 , an execution unit 112 , and an internal bus 111 for executing instructions. Of course, the processor 110 1 contains additional circuitry. The decode unit 116 , registers 114 and execution unit 112 are coupled together by the internal bus 111 . The bus controller 113 provides interface to the host bus 115 and an external cache 132 (e.g., L 2 cache). The decode unit 116 is used for decoding instructions received by processor 110 into control signals and/or microcode entry points. In response to these control signals and/or microcode entry points, the execution unit 112 performs the appropriate operations. The decode unit 116 may be implemented using any number of different mechanisms (e.g., a look-up table, a hardware implementation, a PLA, software translation routines, or a combination of hardware and software translation, etc.). While the decoding of the various instructions is represented herein by a series of if/then statements, it is understood that the execution of an instruction does not require a serial processing of these if/then statements. Rather, any mechanism for logically performing this if/then processing is considered to be within the scope of the implementation. The memory cluster 230 includes a L 1 cache controller 250 , which contains a control register 510 . The control register 510 has two mode selection bits pre-serialization mode (MPRE) and post-serialization mode (MPOST).

The decode unit 116 is shown including packed data instruction set 118 for performing operations on packed data. In one embodiment, the packed data instruction set 118 includes the following instructions: a load fence, a move packed aligned, a move packed unaligned, and a move scalar. In addition to the packed data instructions, the processor 110 1 can include new instructions and/or instructions similar to or the same as those found in existing general purpose processors. For example, in one embodiment the processor 110 1 supports an instruction set which is compatible with the Intel Architecture instruction set used by existing processors, such as the Pentium® processor manufactured by Intel Corporation of Santa Clara, Calif. Alternative embodiments may contain more or less, as well as different, packed data instructions.

The registers 114 represent a storage area on processor 110 1 for storing information, including control/status information, integer data, floating point data, and packed data. The storage area used for storing the packed data is not critical. The term data processing system is used herein to refer to any machine for processing data, including the computer systems(s) described with reference to FIG. 1 .

FIG. 3 is a diagram illustrating the relationship and data paths between the reservation station 210 and the various memory control elements. It is assumed that the decoded instructions are memory-referencing instructions.

As shown in FIG. 3 , the reservation station dispatches the instruction to the memory ordering unit 240 via the Load Bus 312 or the Store Bus 316 if the instruction is a load or store instruction, respectively. The memory ordering unit 240 includes a load buffer unit 322 and a store buffer unit 326 . The load buffer unit 322 receives the load instructions via the load bus 312 . The load buffer unit 322 receives the loads and the load fence (LFENCE) instruction to synchronize the weakly ordered loads.

Instructions in the load buffer unit 322 and store buffer unit 326 are dispatched to the L 1 cache controller 250 . The L 1 cache controller 250 includes a hit/miss detection logic 340 , a L 1 cache 360 , a L 1 cache controller buffer 370 , and a write-back selector 380 . The hit/miss detection logic 340 detects if the load or store instruction hits the L 1 cache or any other L 1 cache controller array or buffer structure. If the instruction is a cacheable request and has a L 1 hit, the cache access is provided via a bus 350 A to the L 1 cache 360 . If the instruction is an uncacheable request, a special cycle, or cacheable request with a L 1 miss, the request is routed to the L 1 cache controller buffer 370 via a bus 350 B. For a cacheable request with a L 1 miss, the missing data is transferred to the L 1 cache 360 from the L 1 cache controller buffer unit 370 via path A. This data typically comes from the L 2 cache or the external memory.

The bus controller 113 services the requests from the L 1 cache controller buffer 370 via path B. For cacheable request misses or uncacheable loads, the data flow is from the bus controller 260 to the L 1 cache controller buffer 370 . For uncacheable stores or writes, the data flow is from the L 1 cache controller buffer 370 to the bus controller 113 .

For loads, data are written back to the core, i.e., the re-order buffer and register file 220 . The write-back data may come from the L 1 cache 360 (for cache hits) or the L 1 cache controller buffer 370 (for cacheable misses or uncacheable loads). The selector 380 switches between these two write-back sources to the write-back bus 255 . The write-back data are then written to the re-order buffer and register file 220 and the reservation station, and the write-back data valid bit is written to the re-order buffer.

FIG. 4 is a diagram illustrating the data flow between different elements in the processor. As shown in FIG. 4 , the data flow involves the reservation station 210 , the memory ordering unit 240 , the L 1 cache controller 250 , and the bus controller 113 .

From the reservation station 210 to the memory ordering unit 240 , the data flow includes a load, a store, and a special cycle. These data requests are dispatched from the processor core. At the memory ordering unit 240 , all loads (or reads) are stored in the load buffer unit, all stores (or writes) are stored in the store buffer unit, all load fence instructions are stored with other loads in the load buffer unit.

The memory operations involved in the loads, stores, or load fences are dispatched from the memory ordering unit 240 to the L 1 cache controller 250 based on memory ordering dependencies. At the L 1 cache controller 250 , the L 1 cache controller buffer services cache misses, uncacheable requests, write combining writes, and certain special cycles. The execution of the load fence instruction is considered a special cycle. For cache hits by cacheable operations, the L 1 cache is accessed.

If the memory operation is satisfied from the L 2 cache or the main memory, the L 1 cache controller buffer send requests to the bus controller 113 . At the bus controller 113 , requests are sent to lower level caches such as the L 2 cache, or to the main memory.

Theory of the Load Fencing Mechanism

Fencing is a mechanism used to ensure synchronization between the processor core and consumer hardware when weakly ordered load instructions are used. The LOAD FENCE (LFENCE) instruction provides an efficient way of ensuring ordering between instruction streams that produce weakly ordered results.

In one embodiment, the LFENCE instruction has no associated data field. The re-order buffer ignores the data on the writeback bus at the cycle of retirement for a LFENCE macro-instruction. In addition, the LFENCE instruction is treated as a no-operation (NOP) by the L 1 CC (i.e., there is no change in its state). To facilitate synchronization, the LFENCE instruction is dispatched to the L 1 CC only after all instructions preceding the LFENCE instruction have already been dispatched and accepted by the L 1 CC.

The LFENCE mechanism is supported by two serialization techniques: pre-serialization and post-serialization. The pre-serialization and post-serialization techniques include those operations that are in program order before and after, respectively, the LFENCE instruction. Both the pre-serialization and post-serialization provide an option to select a default behavior and an enhanced behavior.

FIG. 5 is a diagram illustrating a control register 510 in the L 1 cache controller according to one embodiment. The control registers 510 stores a number of bits to control various operations of the L 1 cache controller. In one embodiment, the control register 510 includes 32 bits. As shown in FIG. 5 , the control register 510 includes a pre-serialization mode (MPRE) bit 512 and a post-serialization mode (MPOST) bit 514 .

The MPRE bit 512 is used to select the mode of pre-serialization. In one embodiment, when the MPRE bit is cleared (i.e., 0 ), the default behavior mode is selected; when the MPRE bit is set (i.e., 1 ), the enhanced behavior mode is selected.

The MPOST bit 514 is used to select the mode of the post-serialization techniques. In one embodiment, when the MPOST bit is cleared (i.e., 0 ), the default behavior mode is selected; when the MPOST bit is set (i.e., 1 ), the enhanced behavior mode is selected.

Pre-serialization

Pre-serialization relates to the operations before the LFENCE instruction. When pre-serialization is enabled, the LFENCE instruction fences load operations. In the L 1 CC, there are a number of buffers that can service various types of memory accesses. The memory accesses relevant to the LFENCE include load operations.

For load operations prior to the LFENCE instruction, the servicing buffers need to be globally observed before the LFENCE is accepted by the L 1 CC. The retirement of the LFENCE is gated (or conditioned) upon the previous load operations being globally observed. The steps after being globally observed for loads to be completed include reading the data to an appropriate register and subsequent de-allocation from the buffers in the L 1 CC. However, other steps in other embodiments may be implemented.

In pre-serialization, outstanding stores do not block the LFENCE and loads need to be globally observed before the LFENCE can be accepted by the L 1 CC. In other words, outstanding stores do not block the LFENCE and all loads need to be globally observed before the LFENCE can be accepted by the L 1 CC.

Post-serialization

Post-serialization relates to the operations subsequent to the LFENCE. Two techniques may be used, one for default behavior and one for enhanced behavior. The mode is controlled by the post-serialization mode bit (MPOST) in the control register as shown in FIG. 5 .

In the default behavior, instructions following the LFENCE in program order are blocked from being dispatched to the L 1 CC until the L 1 CC accepts the LFENCE. Coupled with pre-serialization, this implies that all instructions preceding the LFENCE in program order are globally observed before any instruction which follows the LFENCE in program order is globally observed.

In the enhanced behavior, stores subsequent to the LFENCE are allowed to be executed out of order with respect to the LFENCE. This behavior usually enhances the performance because the stores do not have to wait for the LFENCE to be completed.

FIG. 6 is a flowchart illustrating a process P 600 for synchronizing load operations according to one embodiment. It is assumed that the LFENCE has been allocated in the load buffer in the memory ordering unit (MOU) and all previous instructions have been dispatched and accepted by the L 1 CC.

Upon START, the process P 600 enters block B 610 that dispatches the LFENCE instruction from the memory ordering unit (MOU) to the L 1 cache controller (L 1 CC). Then the process P 600 enters block B 620 to determine if all outstanding loads in the L 1 CC buffers are globally observed. If all outstanding loads in the L 1 CC buffers are not globally observed, the process P 600 enters block B 630 . In block B 630 , the L 1 CC blocks the LFENCE indicating that load instructions prior to the LFENCE have not been globally observed. It should be noted that non-globally observed stores do not block the LFENCE. Upon receipt of the blocking signal from the L 1 CC, the MOU will attempt to re-dispatch the LFENCE in the next available memory pipeline as shown in block B 610 .

If all outstanding loads in the L 1 CC buffers have been globally observed and post-serialization is in enhanced mode B 640 , then instructions following LFENCE may not be executed out of order with respect to LFENCE B 650 . Otherwise, instructions following LFENCE may be executed out of order with respect to LFENCE B 655 .

In block B 660 , the LFENCE is accepted by the L 1 CC and the L 1 CC does not send any blocking information to the MOU buffer. Then the process P 600 enters block B 670 to retire the LFENCE and de-allocates the load buffer in the MOU. The process P 600 then terminates.

Thus, a method and apparatus for synchronizing load operations is disclosed. While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art upon studying this disclosure.