[PATCH v7 15/15] KVM: x86/cpuid: Advise Arch LBR feature in CPUID

[Yang Weijiang]

Add Arch LBR feature bit in CPU cap-mask to expose the feature.
Currently only max LBR depth is available 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>
---
 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 03025eea1524..1ee3014b8977 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -202,6 +202,16 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 		best->ecx |= XFEATURE_MASK_FPSSE;
 	}
 
+	best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
+	if (best) {
+		unsigned int eax, ebx, ecx, edx;
+
+		/* Make sure guest only sees the supported arch lbr depth. */
+		cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx);
+		best->eax &= ~0xff;
+		best->eax |= BIT(fls(eax & 0xff) - 1);
+	}
+
 	kvm_update_pv_runtime(vcpu);
 
 	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
@@ -490,7 +500,7 @@ void kvm_set_cpu_caps(void)
 		F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(SPEC_CTRL) |
 		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(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) | F(ARCH_LBR)
 	);
 
 	/* TSC_ADJUST and ARCH_CAPABILITIES are emulated in software. */
@@ -903,6 +913,27 @@ 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;
+	}
 	case KVM_CPUID_SIGNATURE: {
 		static const char signature[12] = "KVMKVMKVM\0\0";
 		const u32 *sigptr = (const u32 *)signature;
-- 
2.25.1