Re: [PATCH v2 1/4] KVM: vmx/pmu: Add MSR_ARCH_LBR_DEPTH emulation for Arch LBR

[Like Xu]

On 2021/3/2 6:34, Sean Christopherson wrote:
> On Wed, Feb 03, 2021, Like Xu wrote:
> > @@ -348,10 +352,26 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
> >   	return true;
> >   }
> >   
> > +/*
> > + * Check if the requested depth values is supported
> > + * based on the bits [0:7] of the guest cpuid.1c.eax.
> > + */
> > +static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
> > +{
> > +	struct kvm_cpuid_entry2 *best;
> > +
> > +	best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
> > +	if (depth && best)
>
> > +		return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));
>
> I believe this will genereate undefined behavior if depth > 64.  Or if depth < 8.
> And I believe this check also needs to enforce that depth is a multiple of 8.
>
>     For each bit n set in this field, the IA32_LBR_DEPTH.DEPTH value 8*(n+1) is
>     supported.
>
> Thus it's impossible for 0-7, 9-15, etc... to be legal depths.

Thank you! How about:

	best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
	if (best && depth && !(depth % 8))
		return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));

	return false;

> > +
> > +	return false;
> > +}
> > +