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    SubjectRe: [PATCH 1/2] x86: eliminate TS_XSAVE
    On 05/04/2010 11:15 AM, Suresh Siddha wrote:
    > On Tue, 2010-05-04 at 00:41 -0700, Avi Kivity wrote:
    >> On 05/04/2010 12:45 AM, H. Peter Anvin wrote:
    >>>> I was trying to avoid a performance regression relative to the current
    >>>> code, as it appears that some care was taken to avoid the memory reference.
    >>>> I agree that it's probably negligible compared to the save/restore
    >>>> code. If the x86 maintainers agree as well, I'll replace it with
    >>>> cpu_has_xsave.
    >>> I asked Suresh to comment on this, since he wrote the original code. He
    >>> did confirm that the intent was to avoid a global memory reference.
    >> Ok, so you're happy with the patch as is?
    > As use_xsave() is in the hot context switch path, I would like to go
    > with Avi's proposal.

    I would tend to agree. Saving a likely cache miss in the hot context
    switch path is worthwhile.

    I would like to request one change, however. I would like to see the
    alternatives code to be:

    movb $0,reg
    movb $1,reg

    ... instead of using xor (which has to be padded with NOPs, which is of
    course pointless since the slot is a fixed size.) I would suggest using
    a byte-sized variable instead of a dword-size variable to save a few
    bytes, too.

    Once the jump label framework is integrated and has matured, I think we
    should consider using it to save the mov/test/jump.


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