[PATCH v2 15/15] KVM: x86/cpuid: Advertise Arch LBR feature in CPUID

[Yang Weijiang]

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@xxxxxxxxxxxxxxx>
Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
---
 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;
+		}
+	}
 
 	/*
 	 * 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)) {
-- 
2.27.0