Re: [PATCH -v2] kvm: Emulate MOVBE

[Gleb Natapov <gleb@redhat.com>]

On Sun, Apr 14, 2013 at 07:32:16PM +0200, Borislav Petkov wrote:
> 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."?
> 
I am contemplating this, yes.

> > 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.
> 
This is not true for all emulated CPUID bits. X2APIC is emulated and it
is preferable for a guest to use it for example.

> 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?
> 
Yes, you are. I am considering such interface. Adding new specialized interfaces
is last resort though. I am open to other ideas.

--
			Gleb.