Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host

[Paolo Bonzini]

On 26/11/2014 14:53, Radim KrÄmÃÅ wrote:
>>> > > get_xsave = native_xrstor(guest_xsave);  xsave(aligned_userspace_buffer)
>>> > > set_xsave = xrstor(aligned_userspace_buffer);  native_xsave(guest_xsave)
>>> > > 
>>> > > Could that work?
>> > 
>> > It could, though it is more like
>> > 
>> >    get_fpu()
>> >    native_xrstor(guest_xsave)
>> >    xsave(buffer)
>> >    put_fpu()
>> > 
>> > and vice versa.  Also, the userspace buffer is mos likely not aligned,
>> > so you need some kind of bounce buffer.  It can be done if the CPUID
>> > turns out to be a bottleneck, apart from that it'd most likely be slower.
> Yeah, it was mostly making this code more future-proof ... it is easier
> to convince xsave.h to export its structures if CPUID is the problem.
> (I still see some hope for Linux, so performance isn't my primary goal.)
> 
> I'm quite interested in CPUID now though, so I'll try to benchmark it,
> someday.

I'm not sure what is more future proof. :)  I wonder if native_xrstor
could be a problem the day XRSTORS actually sets/restores MSRs as the
processor documentation promises.  We do not need that to pass them to
userspace via KVM_GET/SET_XSAVE because we have KVM_GET/SET_MSR for
that, but it may cause problems if get_xsave uses XRSTORS and thus sets
the MSRs to unanticipated values.  Difficult to say without more
information on Intel's plans.

Paolo
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