Re: [PATCH RFC v1 08/20] KVM: VMX: Support extended register index in exit handling

[Paolo Bonzini]

On 11/14/25 00:22, Chang S. Bae wrote:
> > > @@ -415,7 +420,10 @@ static __always_inline unsigned long 
> > > vmx_get_exit_qual(struct kvm_vcpu *vcpu)
> > >   static inline int vmx_get_exit_qual_gpr(struct kvm_vcpu *vcpu)
> > >   {
> > > -    return (vmx_get_exit_qual(vcpu) >> 8) & 0xf;
> > > +    if (vmx_egpr_enabled(vcpu))
> > > +        return (vmx_get_exit_qual(vcpu) >> 8) & 0x1f;
> > > +    else
> > > +        return (vmx_get_exit_qual(vcpu) >> 8) & 0xf;
> >
> > Can this likewise mask against 0x1f, unconditionally?
>
> It looks like the behavior of that previously-undefined bit is not
> guaranteed -- there's no architectural promise that the bit will always
> read as zero. So in this case, I think it's still safer to rely on the
> enumeration.
The manual says "VM Exit qualification is extended in-place"... This 
suggests that the wording of the SDM, "Not currently defined", really 
means "currently zero, but may not be *in the future*".

It is indeed safer, but it's a bit sad too. :)  Let's at least use 
either static_cpu_has() or a global __read_mostly variable, instead of a 
more expensive test using the vcpu.

Paolo