Messages in this thread | | | From | Keno Fischer <> | Date | Sun, 17 Jun 2018 12:48:22 -0400 | Subject | Re: [RFC PATCH] x86/arch_prctl: Add ARCH_SET_XCR0 to mask XCR0 per-thread |
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> Patch seems pointless if you can already control CPUID, which rr > supports. Just disable AVX512 in CPUID. All code using AVX should check > cpuid (or will fail anyways).
Unfortunately, that is insufficient. Almost difference in CPU behavior between the replayer and the replayee. In particular, the problems for rr here are 1) xgetbv, which can introduce differing register contents (and if code branches on this, potentially differences in control flow). 2) xsave writing more memory than the trace expects, causing divergence in the memory contents of the process.
Robert O'Callahan has a blog post that goes into some detail here: https://robert.ocallahan.org/2018/04/cpuid-features-xsave-and-rr-trace.html
I hope that makes sense. Please let me know if you'd like me to explain the problem in more detail or give an example. FWIW, I do have a version of rr that uses the proposed feature in this patch and it does fix our problem with replaying AVX traces on skylake machines. Without this patch, rr does emulate the cpuid response of the recording machine, but the replay fails because of the mentioned issue.
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