Recording a cache coherency protocol trace for use with a separate memory value trace
A processor that performs cache-based tracing based on recording one or more cache coherency protocol (CCP) messages into a first trace. Based on detecting a memory access to a target memory address, the processor logs into the first trace information usable to obtain a memory value corresponding to the particular memory address from the memory snapshot(s) stored within the second trace. This includes logging the particular memory address, as well as CCP message(s) indicating at least of: (i) that none of a plurality of processing units possessed a first cache line within the cache that overlaps with the target memory address; (ii) that a first processing unit initiated a cache miss for the target memory address; or (iii) that the first processing unit obtained, from a second processing, a second cache line within the cache that overlaps with the target memory address.
1. A processor configured to participate in recording a replayable execution trace based on recording one or more cache coherency protocol (CCP) messages into a first trace, the CCP messages being usable to obtain memory values from one or more memory snapshots stored within a second trace, the processor comprising:
a plurality of processing units;
a cache; and
control logic that causes the processor to at least:
detect a memory access by a first processing unit of the plurality of processing units, the memory access being targeted at a particular memory address during execution of an execution context; and
based on detecting the memory access,
determine that logging is enabled for a memory region corresponding to the particular memory address;
based on logging being enabled for the memory region, determine to log information usable to obtain a memory value corresponding to the particular memory address from the one or more memory snapshots stored within the second trace; and
log, into the first trace, the information usable to obtain the memory value corresponding to the particular memory address from the one or more memory snapshots stored within the second trace, including logging the particular memory address and at least one of:
a first CCP message indicating that none of the plurality of processing units possessed a first cache line within the cache that overlaps with the particular memory address;
a second CCP message indicating that the first processing unit initiated a cache miss for the particular memory address; or
a third CCP message indicating that the first processing unit obtained, from a second processing unit of the plurality of processing units, a second cache line within the cache that overlaps with the particular memory address.
2. The processor of claim 1 , wherein the control logic causes the processor to log the first CCP message indicating that none of the plurality of processing units possessed the first cache line within the cache that overlaps with the particular memory address.
3. The processor of claim 1 , wherein the control logic causes the processor to log the second CCP message indicating that the first processing unit initiated the cache miss for the particular memory address.
4. The processor of claim 1 , wherein the control logic causes the processor to log the third CCP message indicating that the first processing unit obtained, from the second processing unit, the second cache line within the cache that overlaps with the particular memory address.
5. The processor of claim 1 , wherein the control logic also causes the processor to log, into the first trace, one or more CCP messages indicating that one or more of the plurality of processing units caused one or more evictions from the cache.
6. The processor of claim 1 , wherein the memory access comprises a first memory access, and wherein the control logic also causes the processor to:
detect a second memory access during execution of the execution context;
determine that the second memory access is a read that is targeted to an uncached memory location; and
based on the second memory access being the read that is targeted to the uncached memory location, log at least a value read by the second memory access.
7. The processor of claim 6 , wherein the first trace comprises a first trace data stream, and wherein logging the value read by the second memory access comprises at least one of (i) logging the value into an encrypted second trace data stream, or encrypting the value within the first trace data stream.
8. The processor of claim 1 , wherein the control logic also causes the processor to log, into the first trace, a side-effect of execution of at least one non-deterministic processor instruction of the execution context.
9. The processor of claim 1 , wherein the control logic also causes the processor to determine that logging is enabled for the execution context, and wherein the processor logs the information usable to obtain the memory value corresponding to the particular memory address from the one or more memory snapshots stored within the second trace based at least on logging being enabled for the execution context.
10. The processor of claim 1 , wherein the execution context comprises a first execution context, and wherein, based at least on a second execution context initiating a write into a memory space of the first execution context, the control logic also causes the processor to perform at least one of:
log at least one of a memory address or a value of the write;
evict a cache line overlapping with the memory address of the write;
mark a memory page corresponding to the memory address of the write as needing to be logged; or
initiate a trap handler that logs the memory page corresponding to the memory address.
11. The processor of claim 10 , wherein the control logic causes the processor to log the value of the write, wherein the first trace comprises a first trace data stream, and wherein logging the value of the write comprises at least one of (i) logging the value of the write into an encrypted second trace data stream, or encrypting the value of the write within the first trace data stream.
12. The processor of claim 1 , wherein the control logic also causes the processor to enable trace recording for the execution context.
13. The processor of claim 12 , wherein the control logic also causes the processor to flush, from the cache, at least one cache line that overlaps with a memory space of the execution context in connection with enabling trace recording for the execution context.
14. The processor of claim 1 , wherein the memory access comprises a first memory access, and wherein the control logic also causes the processor to:
detect a third memory access during execution of the execution context;
determine that the third memory access is targeted to a read-only memory location; and
based on the third memory access being targeted to a read-only memory location, refrain from logging a CCP message or an uncached read for the third memory access.
15. The processor of claim 1 , wherein,
determining that logging is enabled for the memory region comprises determining that the memory region associated is tagged to be logged; and
determining that the memory region associated is tagged to be logged comprises identifying one or more of a flag in a page table entry or a page directory entry, a mapping stored in a data structure in a system memory, or a mapping stored in a register of the processor.
16. A method, implemented at a processor that includes a plurality of processing units and a cache, for participating in recording a replayable execution trace based on recording one or more cache coherency protocol (CCP) messages into a first trace, the CCP messages being usable to obtain memory values from one or more memory snapshots stored within a second trace, the method comprising:
detecting a memory access by a first processing unit of the plurality of processing units, the memory access being targeted at a particular memory address during execution of an execution context; and
based on detecting the memory access,
determining that logging is enabled for a memory region corresponding to the particular memory address;
based on logging being enabled for the memory region, determining to log information usable to obtain a memory value corresponding to the particular memory address from the one or more memory snapshots stored within the second trace; and
logging, into the first trace, the information usable to obtain the memory value corresponding to the particular memory address from the one or more memory snapshots stored within the second trace, including logging the particular memory address and at least one of:
a first CCP message indicating that none of the plurality of processing units possessed a first cache line within the cache that overlaps with the particular memory address;
a second CCP message indicating that the first processing unit initiated a cache miss for the particular memory address; or
a third CCP message indicating that the first processing unit obtained, from a second processing unit of the plurality of processing units, a second cache line within the cache that overlaps with the particular memory address.
17. The method of claim 16 , wherein the method comprises logging the first CCP message indicating that none of the plurality of processing units possessed the first cache line within the cache that overlaps with the particular memory address.
18. The method of claim 16 , wherein the method comprises logging the second CCP message indicating that the first processing unit initiated the cache miss for the particular memory address.
19. The method of claim 16 , wherein the method comprises logging the third CCP message indicating that the first processing unit obtained, from the second processing unit, the second cache line within the cache that overlaps with the particular memory address.
20. The method of claim 16 , further comprising logging, into the first trace, one or more CCP messages indicating that one or more of the plurality of processing units caused one or more evictions from the cache.