KVM: x86: Snapshot if a vCPU's vendor model is AMD vs. Intel compatible

commit fd706c9b1674e2858766bfbf7430534c2b26fbef upstream.

Add kvm_vcpu_arch.is_amd_compatible to cache if a vCPU's vendor model is
compatible with AMD, i.e. if the vCPU vendor is AMD or Hygon, along with
helpers to check if a vCPU is compatible AMD vs. Intel.  To handle Intel
vs. AMD behavior related to masking the LVTPC entry, KVM will need to
check for vendor compatibility on every PMI injection, i.e. querying for
AMD will soon be a moderately hot path.

Note!  This subtly (or maybe not-so-subtly) makes "Intel compatible" KVM's
default behavior, both if userspace omits (or never sets) CPUID 0x0 and if
userspace sets a completely unknown vendor.  One could argue that KVM
should treat such vCPUs as not being compatible with Intel *or* AMD, but
that would add useless complexity to KVM.

KVM needs to do *something* in the face of vendor specific behavior, and
so unless KVM conjured up a magic third option, choosing to treat unknown
vendors as neither Intel nor AMD means that checks on AMD compatibility
would yield Intel behavior, and checks for Intel compatibility would yield
AMD behavior.  And that's far worse as it would effectively yield random
behavior depending on whether KVM checked for AMD vs. Intel vs. !AMD vs.
!Intel.  And practically speaking, all x86 CPUs follow either Intel or AMD
architecture, i.e. "supporting" an unknown third architecture adds no
value.

Deliberately don't convert any of the existing guest_cpuid_is_intel()
checks, as the Intel side of things is messier due to some flows explicitly
checking for exactly vendor==Intel, versus some flows assuming anything
that isn't "AMD compatible" gets Intel behavior.  The Intel code will be
cleaned up in the future.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240405235603.1173076-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

5 files changed, 14 insertions, 2 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index dfcdcafe3a2c..887a171488ea 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -773,6 +773,7 @@ struct kvm_vcpu_arch {
 	int cpuid_nent;
 	struct kvm_cpuid_entry2 *cpuid_entries;
 	u32 kvm_cpuid_base;
+	bool is_amd_compatible;
 
 	u64 reserved_gpa_bits;
 	int maxphyaddr;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 62a44455c51d..f02961cbbb75 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -340,6 +340,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 
 	kvm_update_pv_runtime(vcpu);
 
+	vcpu->arch.is_amd_compatible = guest_cpuid_is_amd_or_hygon(vcpu);
 	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
 	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
 
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index b1658c0de847..18fd2e845989 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -125,6 +125,16 @@ static inline bool guest_cpuid_is_intel(struct kvm_vcpu *vcpu)
 	return best && is_guest_vendor_intel(best->ebx, best->ecx, best->edx);
 }
 
+static inline bool guest_cpuid_is_amd_compatible(struct kvm_vcpu *vcpu)
+{
+	return vcpu->arch.is_amd_compatible;
+}
+
+static inline bool guest_cpuid_is_intel_compatible(struct kvm_vcpu *vcpu)
+{
+	return !guest_cpuid_is_amd_compatible(vcpu);
+}
+
 static inline int guest_cpuid_family(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d30325e297a0..13134954e24d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4649,7 +4649,7 @@ static void reset_guest_rsvds_bits_mask(struct kvm_vcpu *vcpu,
 				context->cpu_role.base.level, is_efer_nx(context),
 				guest_can_use_gbpages(vcpu),
 				is_cr4_pse(context),
-				guest_cpuid_is_amd_or_hygon(vcpu));
+				guest_cpuid_is_amd_compatible(vcpu));
 }
 
 static void
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f724765032bc..a2ea636a2308 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3278,7 +3278,7 @@ static bool is_mci_status_msr(u32 msr)
 static bool can_set_mci_status(struct kvm_vcpu *vcpu)
 {
 	/* McStatusWrEn enabled? */
-	if (guest_cpuid_is_amd_or_hygon(vcpu))
+	if (guest_cpuid_is_amd_compatible(vcpu))
 		return !!(vcpu->arch.msr_hwcr & BIT_ULL(18));
 
 	return false;