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Re: [Xen-devel] [PATCH] x86/xsave: simplify xcomp_bv initialization

On 17/12/15 16:58, Jan Beulich wrote:
> This eliminates a number of pointless conditionals: Bits 0 and 1 of
> xcomp_bv don't matter anyway, and as long as none of bits 2..62 are
> set, setting bit 63 is pointless too.

xcomp_bv[63] is the indication of whether the area is compressed or not.

It will be set by the processor when an XSAVEC instruction is issued,
and until that point, it doesn't particularly matter as no bits are set.

However, it is an ABI indication of the size of the xsave area.  It
turns out that we are redundantly allocating an uncompressed xsave, even
when we will only used the compressed form.

For now, I don't think there is any harm, as the lack of
XSTATE_COMPACTION_ENABLED causes the size indication to actually be
correct, but it will become necessary when we start making use of the
optimisations that compaction allows.

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

(On that note, I really should make some patches for my planned cleanup
for XSA-165.  I have just been too busy recently.)

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