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    SubjectRe: [RFC] x86: xsave/xrstor support, ucontext_t extensions
    H. Peter Anvin wrote:
    > Andi Kleen wrote:
    >>> Ok. CPU folks are planning to make some of the bytes at the end of
    >>> fxsave
    >>> image, SW usable.
    >> Are they always zeroed in earlier CPUs though? If not that wouldn't
    >> work 100% reliably because whatever cookie you put in could have been
    >> there before by chance.
    >> I don't see anything in the SDM guaranteeing zeroing.
    > I'm pretty sure they weren't zeroed by the CPUs. If they weren't zeroed
    > *by the kernel*, there might have been an information leak.

    I don't think there is one. We never copy fxsave completely out of the
    kernel. x86-64 does FXSAVE directly in/out user space, but the
    only leak is what there was before.


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