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    SubjectRe: [RFC PATCH] x86/arch_prctl: Add ARCH_SET_XCR0 to mask XCR0 per-thread
    > 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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