Re: [PATCH v11 16/16] KVM: x86/cpuid: Advertise Arch LBR feature in CPUID

["Liang, Kan" <kan.liang@linux.intel.com>]

On 5/5/2022 11:33 PM, Yang Weijiang wrote:
> Add Arch LBR feature bit in CPU cap-mask to expose the feature.
> Only max LBR depth is supported for guest, and it's consistent
> with host Arch LBR settings.
> 
> Co-developed-by: Like Xu <like.xu@linux.intel.com>
> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>

Reviewed-by: Kan Liang <kan.liang@linux.intel.com>

> ---
>   arch/x86/kvm/cpuid.c | 33 ++++++++++++++++++++++++++++++++-
>   1 file changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 63870f78ed16..be4eb4e5e1fc 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -102,6 +102,16 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
>   		if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
>   			return -EINVAL;
>   	}
> +	best = cpuid_entry2_find(entries, nent, 0x1c, 0);
> +	if (best) {
> +		unsigned int eax, ebx, ecx, edx;
> +
> +		/* Reject user-space CPUID if depth is different from host's.*/
> +		cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx);
> +
> +		if ((best->eax & 0xff) != BIT(fls(eax & 0xff) - 1))
> +			return -EINVAL;
> +	}
>   
>   	/*
>   	 * Exposing dynamic xfeatures to the guest requires additional
> @@ -598,7 +608,7 @@ void kvm_set_cpu_caps(void)
>   		F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
>   		F(MD_CLEAR) | F(AVX512_VP2INTERSECT) | F(FSRM) |
>   		F(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) |
> -		F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16)
> +		F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(ARCH_LBR)
>   	);
>   
>   	/* TSC_ADJUST and ARCH_CAPABILITIES are emulated in software. */
> @@ -1044,6 +1054,27 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   				goto out;
>   		}
>   		break;
> +	/* Architectural LBR */
> +	case 0x1c: {
> +		u32 lbr_depth_mask = entry->eax & 0xff;
> +
> +		if (!lbr_depth_mask ||
> +		    !kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR)) {
> +			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
> +			break;
> +		}
> +		/*
> +		 * KVM only exposes the maximum supported depth, which is the
> +		 * fixed value used on the host side.
> +		 * KVM doesn't allow VMM userspace to adjust LBR depth because
> +		 * guest LBR emulation depends on the configuration of host LBR
> +		 * driver.
> +		 */
> +		lbr_depth_mask = BIT((fls(lbr_depth_mask) - 1));
> +		entry->eax &= ~0xff;
> +		entry->eax |= lbr_depth_mask;
> +		break;
> +	}
>   	/* Intel AMX TILE */
>   	case 0x1d:
>   		if (!kvm_cpu_cap_has(X86_FEATURE_AMX_TILE)) {