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xen-devel
Re: [Xen-devel] [PATCH 0/3 v2] XSAVE/XRSTOR fixes and enhancements
Keir Fraser wrote:
On 01/09/2010 02:53, "Weidong Han" <weidong.han@xxxxxxxxx> wrote:
Performance overhead of this fix? Is there no other lazy save technique that
can work?
I think the cost of set_xcr0 which just changes some bits in XCR0
register should be little. I don't have any optimization for it now.
I obviously don't mean that. What about the increased cost of XSAVE and
XRSTOR s/r'ing more state unconditionally? At least it is conditional on
v->fpu_dirtied I suppose?
One possible optimization is that only save/restore legacy states (FPU
and SSE) for guests which don't enable XSAVE.
Both xsave() and xrstor are invoked only when v->fpu_dirtied.
Patch 3/3. Enable guest AVX
This patch enables Intel(R) Advanced Vector Extension (AVX) for guest.
If we enable this but don't implement save/restore then don't guests lose
state across s/r with unpredictable results?
Yes. As I said in another email, actually it already breaks hvm guests
save/restore on platforms which supports XSAVE/XRSTOR.
Wow, so the last couple of Xen releases are broken for the latest Intel
platforms unless you specify no-xsave. Handy to know I guess.
Why is the feature flag stuff all stuffed in Xen itself rather than
xc_cpuid_x86.c, by the way? Shouldn't your change also be in the same place,
or (much preferably) all XSAVE related stuff be moved out into
libxc/xc_cpuid_x86.c?
I'm afraid XSAVE related stuff cannot be move out into
libxc/xc_cpuid_x86.c completely? At least Xen needs to control
X86_CR4_OSXSAVE for guests which don't support XSAVE. I will have a look
at it.
Regards,
Weidong
-- Keir
Regards,
Weidong
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