[v2,15/15] KVM: x86/cpuid: Advertise Arch LBR feature in CPUID
Commit Message
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 | 36 +++++++++++++++++++++++++++++++++++-
1 file changed, 35 insertions(+), 1 deletion(-)
Comments
On 25/11/2022 12:06 pm, Yang Weijiang wrote:
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 85e3df6217af..60b3c591d462 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -134,6 +134,19 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
> if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
> return -EINVAL;
> }
> + if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR)) {
> + 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.*/
Try to verify this in the KVM selftest, as this behavior is different from the host.
> + cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx); > +
> + if ((eax & 0xff) &&
> + (best->eax & 0xff) != BIT(fls(eax & 0xff) - 1))
> + return -EINVAL;
> + }
> + }
On 12/22/2022 7:03 PM, Like Xu wrote:
> On 25/11/2022 12:06 pm, Yang Weijiang wrote:
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 85e3df6217af..60b3c591d462 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -134,6 +134,19 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
>> if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
>> return -EINVAL;
>> }
>> + if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR)) {
>> + 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.*/
>
> Try to verify this in the KVM selftest, as this behavior is different
> from the host.
Will do it.
>
>> + cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx); > +
>> + if ((eax & 0xff) &&
>> + (best->eax & 0xff) != BIT(fls(eax & 0xff) - 1))
>> + return -EINVAL;
>> + }
>> + }
On Thu, Nov 24, 2022, 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 | 36 +++++++++++++++++++++++++++++++++++-
> 1 file changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 85e3df6217af..60b3c591d462 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -134,6 +134,19 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
> if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
> return -EINVAL;
> }
> + if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR)) {
> + 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 ((eax & 0xff) &&
> + (best->eax & 0xff) != BIT(fls(eax & 0xff) - 1))
> + return -EINVAL;
> + }
> + }
Drop this. While I think everyone agrees that KVM's CPUID uAPI sucks, the status
quo is to let userspace shoot itself in the foot. I.e. disallow enabling LBRs
with a "bad" config, but don't reject the ioctl().
>
> /*
> * Exposing dynamic xfeatures to the guest requires additional
> @@ -652,7 +665,7 @@ 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)
As mentioned earlier, omit this and make it opt-in.
> );
>
> /* TSC_ADJUST and ARCH_CAPABILITIES are emulated in software. */
> @@ -1074,6 +1087,27 @@ 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));
C'mon. More unnecessary dependencies on perf using the max depth.
> + entry->eax &= ~0xff;
> + entry->eax |= lbr_depth_mask;
> + break;
> + }
> /* Intel AMX TILE */
> case 0x1d:
> if (!kvm_cpu_cap_has(X86_FEATURE_AMX_TILE)) {
> --
> 2.27.0
>
@@ -134,6 +134,19 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
return -EINVAL;
}
+ if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR)) {
+ 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 ((eax & 0xff) &&
+ (best->eax & 0xff) != BIT(fls(eax & 0xff) - 1))
+ return -EINVAL;
+ }
+ }
/*
* Exposing dynamic xfeatures to the guest requires additional
@@ -652,7 +665,7 @@ 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. */
@@ -1074,6 +1087,27 @@ 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)) {