The p6 family processors provide no hardware support

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Unformatted text preview: of 4 KBytes and the size is not 0. Example 9-4. MemTypeSet Pseudocode IF CPU_FEATURES.MTRR (* processor supports MTRRs *) THEN IF BASE and SIZE are not 4-KByte aligned or size is 0 THEN return INVALID; FI; IF (BASE + SIZE) wrap 4-GByte address space THEN return INVALID; 9-29 MEMORY CACHE CONTROL FI; IF TYPE is invalid for P6 family processors THEN return UNSUPPORTED; FI; IF TYPE is WC and not supported THEN return UNSUPPORTED; FI; IF MTRRcap.FIX is set AND range can be mapped using a fixed-range MTRR THEN pre_mtrr_change(); update affected MTRR; post_mtrr_change(); FI; ELSE (* try to map using a variable MTRR pair *) IF MTRRcap.VCNT = 0 THEN return UNSUPPORTED; FI; IF conflicts with current variable ranges THEN return RANGE_OVERLAP; FI; IF no MTRRs available THEN return VAR_NOT_AVAILABLE; FI; IF BASE and SIZE do not meet the power of 2 requirements for variable MTRRs THEN return INVALID_VAR_REQUEST; FI; pre_mtrr_change(); Update affected MTRRs; post_mtrr_change(); FI; pre_mtrr_change() BEGIN disable interrupts; Save current value of CR4; disable and flush caches; flush TLBs; disable MTRRs; IF multiprocessing THEN maintain consistency through IPIs; FI; END post_mtrr_change() BEGIN flush caches and TLBs; enable MTRRs; 9-30 MEMORY CACHE CONTROL enable caches; restore value of CR4; enable interrupts; END The physical address to variable range mapping algorithm in the MemTypeSet function detects conflicts with current variable range registers by cycling through them and determining whether the physical address in question matches any of the current ranges. During this scan, the algorithm can detect whether any current variable ranges overlap and can be concatenated into a single range. The pre_mtrr_change() function disables interrupts prior to changing the MTRRs, to avoid executing code with a partially valid MTRR setup. The algorithm disables caching by setting the CD flag and clearing the NW flag in control register CR0. The caches are invalidated using the WBINVD instruction. The algorithm disables the page global flag (PGE) in control register CR4, if necessary, then flushes all TLB entries by updating control register CR3. Finally, it disables MTRRs by clearing the E flag in the MTRRdefType register. After the memory type is updated, the post_mtrr_change() function re-enables the MTRRs and again invalidates the caches and TLBs. This second invalidation is required because of the processor’s aggressive prefetch of both instructions and data. The algorithm restores interrupts and re-enables caching by setting the CD flag. An operating system can batch multiple MTRR updates so that only a single pair of cache invalidations occur. 9.12.8. Multiple-Processor Considerations In multiple-processor systems, the operating systems must maintain MTRR consistency between all the processors in the system. The P6 family processors provide no hardware support to maintain this consistency. In general, all processors must have the same MTRR values. This requirement implies that when the operating system initializes a multiple-processor system, it must load the MTRRs of the boot processor while the E flag in register MTRRdefType is 0. The operating system then directs other processors to load their MTRRs with the same memory map. After all the processors have loaded their MTRRs, the operating system signals them to enable their MTRRs. Barrier synchronization is used to prevent further memory accesses until all processors indicate that the MTRRs are enabled. This synchronization is likely to be a shootdown style algorithm, with shared variables and interprocessor interrupts. Any change to the value of the MTRRs in a multiple-processor system requires the operating system to repeat the loading and enabling process to maintain consistency, using the following procedure: 1. Broadcast to all processors to execute the following code sequence. 2. Disable interrupts. 3. Wait for all processors to reach this point. 9-31 MEMORY CACHE CONTROL 4. Enter the no-fill cache mode. (Set the CD flag in control register CR0 to 1 and the NW flag to 0.) 5. Flush all caches using the WBINVD instruction. 6. Clear the PGE flag in control register CR4 (if set). 7. Flush all TLBs. (Execute a MOV from control register CR3 to another register and then a MOV from that register back to CR3.) 8. Disable all range registers (by clearing the E flag in register MTRRdefType). If only variable ranges are being modified, software may clear the valid bits for the affected register pairs instead. 9. Update the MTRRs. 10. Enable all range registers (by setting the E flag in register MTRRdefType). If only variable-range registers were modified and their individual valid bits were cleared, then set the valid bits for the affected ranges instead. 11. Flush all caches and all TLBs a second time. (The TLB flush is required for P6 family processors. Executing the WBINVD instruction is not needed when using P6 family processors, but it may be needed in future systems.) 12. Enter the normal cache mode to re-enable caching. (Set the CD and NW flags in control...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at Berkeley.

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