Re: [PATCH 2/2] KVM: nSVM: improve SYSENTER emulation on AMD

[Paolo Bonzini]

On 15/03/21 19:19, Maxim Levitsky wrote:
> On Mon, 2021-03-15 at 18:56 +0100, Paolo Bonzini wrote:
> > On 15/03/21 18:43, Maxim Levitsky wrote:
> > > +	if (!guest_cpuid_is_intel(vcpu)) {
> > > +		/*
> > > +		 * If hardware supports Virtual VMLOAD VMSAVE then enable it
> > > +		 * in VMCB and clear intercepts to avoid #VMEXIT.
> > > +		 */
> > > +		if (vls) {
> > > +			svm_clr_intercept(svm, INTERCEPT_VMLOAD);
> > > +			svm_clr_intercept(svm, INTERCEPT_VMSAVE);
> > > +			svm->vmcb->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
> > > +		}
> > > +		/* No need to intercept these msrs either */
> > > +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_EIP, 1, 1);
> > > +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_ESP, 1, 1);
> > > +	}
> >
> > An "else" is needed here to do the opposite setup (removing the "if
> > (vls)" from init_vmcb).
>
> init_vmcb currently set the INTERCEPT_VMLOAD and INTERCEPT_VMSAVE and it doesn't enable vls

There's also this towards the end of the function:

        /*
         * If hardware supports Virtual VMLOAD VMSAVE then enable it
         * in VMCB and clear intercepts to avoid #VMEXIT.
         */
        if (vls) {
                svm_clr_intercept(svm, INTERCEPT_VMLOAD);
                svm_clr_intercept(svm, INTERCEPT_VMSAVE);
                svm->vmcb->control.virt_ext |= 
VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
        }

> thus there is nothing to do if I don't want to enable vls.
> It seems reasonable to me.
>
> Both msrs I marked as '.always = false' in the
> 'direct_access_msrs', which makes them be intercepted by the default.
> If I were to use '.always = true' it would feel a bit wrong as the intercept is not always
> enabled.

I agree that .always = false is correct.

> What do you think?

You can set the CPUID multiple times, so you could go from AMD to Intel 
and back.

Thanks,

Paolo