Re: [PATCH v3 5/9] KVM: vmx/pmu: Add MSR_ARCH_LBR_DEPTH emulation for Arch LBR

[Sean Christopherson]

On Wed, Mar 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 (best && depth && !(depth % 8))

This is still wrong, it fails to weed out depth > 64.

Not that this is a hot path, but it's probably worth double checking that the
compiler generates simple code for "depth % 8", e.g. it can be "depth & 7)".

> +		return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));
> +
> +	return false;
> +}
> +