Re: [PATCH -v2] kvm: Emulate MOVBE

[Borislav Petkov <bp@alien8.de>]

On Sun, Apr 14, 2013 at 10:41:07AM +0300, Gleb Natapov wrote:
> Currently userspace assumes that that cpuid configuration returned by
> KVM_GET_SUPPORTED_CPUID is the optimal one. What we want here is a way
> for KVM to tell userspace that it can emulate movbe though it is not
> optimal.

Ok, I don't understand.

You want to tell userspace: "yes, we do support a hw feature but we
emulate it."?

> Userspace alone cannot figure it out. It can check host's cpuid
> directly and assume that if cpuid bit is not present on the host cpu,
> but reported as supported by KVM then it is emulated by KVM and this is
> not optimal, but sometimes emulation is actually desirable (x2apic), so
> such assumption is not always correct.

Right, and this is what we have, AFAICT. And if userspace does that what
you exemplify above, you get exactly that - a feature bit not set in
CPUID but KVM reporting it set means, it is emulated. There's no room
for other interpretations here. Which probably means also not optimal
because it is not done in hw.

Or, do you want to have a way to say with KVM_GET_SUPPORTED_CPUID that
"the features I'm reporting to you are those - a subset of them are not
optimally supported because I'm emulating them."

Am I close?

Thanks.

-- 
Regards/Gruss,
    Boris.

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