Re: [PATCH v3 3/4] KVM: x86: Virtualize AMD CPUID faulting

[Sean Christopherson <seanjc@google.com>]

On Thu, May 14, 2026, Jim Mattson wrote:
> On Thu, May 14, 2026 at 7:28 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, May 14, 2026, Jim Mattson wrote:
> > > > diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> > > > index 95d09ccbf951..fc96ba86c644 100644
> > > > --- a/arch/x86/kvm/cpuid.h
> > > > +++ b/arch/x86/kvm/cpuid.h
> > > > @@ -185,8 +185,9 @@ static inline int guest_cpuid_stepping(struct kvm_vcpu *vcpu)
> > > >
> > > >  static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)
> > > >  {
> > > > -       return vcpu->arch.msr_misc_features_enables &
> > > > -                 MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
> > > > +       return (vcpu->arch.msr_misc_features_enables &
> > > > +               MSR_MISC_FEATURES_ENABLES_CPUID_FAULT) ||
> > > > +               (vcpu->arch.msr_hwcr & MSR_K7_HWCR_CPUID_USER_DIS);
> > > >  }
> > >
> > > Sashiko raises a good point here about a pre-existing issue that
> >
> > Calling this pre-existing is a bit of a stretch.  I'm guessing VMX doesn't check
> > the #GP before the VM-Exit (checking #GP before a VM-Exit is so stupid).
> 
> Per the SDM, volume 3, section 27.1.1: Relative Priority of Faults and VM Exits
> 
> Certain exceptions have priority over VM exits. These include invalid-opcode
> exceptions, faults based on privilege level,1 and general-protection
> exceptions that are based on checking I/O permission bits in the task- state
> segment (TSS). For example, execution of RDMSR with CPL = 3 generates a
> general-protection exception and not a VM exit.2

...

> > Where in the APM?  I can't find anything in the description of CPUID or CpuidUserDis
> > that specifies the priority, and "Table 15-7. Instruction Intercepts" is flat out
> > wrong because it just says:
> >
> >    CPUID  CPUID  No exceptions to check.
> 
> APM volume 2, section 15.7: Intercept Operation
> 
> > Generally, instruction intercepts are checked after simple exceptions
> (such as #GP—when CPL is incorrect—or #UD) have been checked, but
> before exceptions related to
> memory accesses (such as page faults) and exceptions based on specific
> operand values.

Oooh, this is based on the generic CPL rules.  I didn't think about it from that
perspective.  So yeah, addressing that does make sense.  What a pain.