Re: [PATCH v5 3/7] KVM: X86: Expose IA32_PKRS MSR

From: Sean Christopherson
Date: Tue Nov 09 2021 - 10:30:18 EST


On Tue, Nov 09, 2021, Chenyi Qiang wrote:
>
> On 11/9/2021 1:44 AM, Sean Christopherson wrote:
> > Hrm. Ideally this would be open coded in vmx_set_msr(). Long term, the RESET/INIT
> > paths should really treat MSR updates as "normal" host_initiated writes instead of
> > having to manually handle every MSR.
> >
> > That would be a bit gross to handle in vmx_vcpu_reset() since it would have to
> > create a struct msr_data (because __kvm_set_msr() isn't exposed to vendor code),
> > but since vcpu->arch.pkrs is relevant to the MMU I think it makes sense to
> > initiate the write from common x86.
> >
> > E.g. this way there's not out-of-band special code, vmx_vcpu_reset() is kept clean,
> > and if/when SVM gains support for PKRS this particular path Just Works. And it would
> > be an easy conversion for my pipe dream plan of handling MSRs at RESET/INIT via a
> > list of MSRs+values.
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ac83d873d65b..55881d13620f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -11147,6 +11147,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> > kvm_set_rflags(vcpu, X86_EFLAGS_FIXED);
> > kvm_rip_write(vcpu, 0xfff0);
> >
> > + if (kvm_cpu_cap_has(X86_FEATURE_PKS))
> > + __kvm_set_msr(vcpu, MSR_IA32_PKRS, 0, true);
> > +
>
> Got it. In addition, is it necessary to add on-INIT check? like:
>
> if (kvm_cpu_cap_has(X86_FEATURE_PKS) && !init_event)
> __kvm_set_msr(vcpu, MSR_IA32_PKRS, 0, true);
>
> PKRS should be preserved on INIT, not cleared. The SDM doesn't make this
> clear either.

Hmm, but your cover letter says:

To help patches review, one missing info in SDM is that PKSR will be
cleared on Powerup/INIT/RESET, which should be listed in Table 9.1
"IA-32 and Intel 64 Processor States Following Power-up, Reset, or INIT"

Which honestly makes me a little happy because I thought I was making stuff up
for a minute :-)

So which is it?