KVM: x86: Add helpers to perform CPUID-based guest vendor check

Add helpers to provide CPUID-based guest vendor checks, i.e. to do the
ugly register comparisons.  Use the new helpers to check for an AMD
guest vendor in guest_cpuid_is_amd() as well as in the existing emulator
flows.

Using the new helpers fixes a _very_ theoretical bug where
guest_cpuid_is_amd() would get a false positive on a non-AMD virtual CPU
with a vendor string beginning with "Auth" due to the previous logic
only checking EBX.  It also fixes a marginally less theoretically bug
where guest_cpuid_is_amd() would incorrectly return false for a guest
CPU with "AMDisbetter!" as its vendor string.

Fixes: a0c0feb5 ("KVM: x86: reserve bit 8 of non-leaf PDPEs and PML4Es in 64-bit mode on AMD")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.h

@@ -219,7 +219,7 @@ static inline bool guest_cpuid_is_amd(struct kvm_vcpu *vcpu)
 	struct kvm_cpuid_entry2 *best;
 
 	best = kvm_find_cpuid_entry(vcpu, 0, 0);
-	return best && best->ebx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx;
+	return best && is_guest_vendor_amd(best->ebx, best->ecx, best->edx);
 }
 
 static inline int guest_cpuid_family(struct kvm_vcpu *vcpu)

arch/x86/kvm/emulate.c

@@ -2723,9 +2723,7 @@ static bool vendor_intel(struct x86_emulate_ctxt *ctxt)
 
 	eax = ecx = 0;
 	ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx, false);
-	return ebx == X86EMUL_CPUID_VENDOR_GenuineIntel_ebx
-		&& ecx == X86EMUL_CPUID_VENDOR_GenuineIntel_ecx
-		&& edx == X86EMUL_CPUID_VENDOR_GenuineIntel_edx;
+	return is_guest_vendor_intel(ebx, ecx, edx);
 }
 
 static bool em_syscall_is_enabled(struct x86_emulate_ctxt *ctxt)
@@ -2744,34 +2742,16 @@ static bool em_syscall_is_enabled(struct x86_emulate_ctxt *ctxt)
 	ecx = 0x00000000;
 	ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx, false);
 	/*
-	 * Intel ("GenuineIntel")
-	 * remark: Intel CPUs only support "syscall" in 64bit
-	 * longmode. Also an 64bit guest with a
-	 * 32bit compat-app running will #UD !! While this
-	 * behaviour can be fixed (by emulating) into AMD
-	 * response - CPUs of AMD can't behave like Intel.
+	 * remark: Intel CPUs only support "syscall" in 64bit longmode. Also a
+	 * 64bit guest with a 32bit compat-app running will #UD !! While this
+	 * behaviour can be fixed (by emulating) into AMD response - CPUs of
+	 * AMD can't behave like Intel.
 	 */
-	if (ebx == X86EMUL_CPUID_VENDOR_GenuineIntel_ebx &&
-	    ecx == X86EMUL_CPUID_VENDOR_GenuineIntel_ecx &&
-	    edx == X86EMUL_CPUID_VENDOR_GenuineIntel_edx)
+	if (is_guest_vendor_intel(ebx, ecx, edx))
 		return false;
 
-	/* AMD ("AuthenticAMD") */
-	if (ebx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx &&
-	    ecx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ecx &&
-	    edx == X86EMUL_CPUID_VENDOR_AuthenticAMD_edx)
-		return true;
-
-	/* AMD ("AMDisbetter!") */
-	if (ebx == X86EMUL_CPUID_VENDOR_AMDisbetterI_ebx &&
-	    ecx == X86EMUL_CPUID_VENDOR_AMDisbetterI_ecx &&
-	    edx == X86EMUL_CPUID_VENDOR_AMDisbetterI_edx)
-		return true;
-
-	/* Hygon ("HygonGenuine") */
-	if (ebx == X86EMUL_CPUID_VENDOR_HygonGenuine_ebx &&
-	    ecx == X86EMUL_CPUID_VENDOR_HygonGenuine_ecx &&
-	    edx == X86EMUL_CPUID_VENDOR_HygonGenuine_edx)
+	if (is_guest_vendor_amd(ebx, ecx, edx) ||
+	    is_guest_vendor_hygon(ebx, ecx, edx))
 		return true;
 
 	/*

arch/x86/kvm/kvm_emulate.h

@@ -396,6 +396,30 @@ struct x86_emulate_ctxt {
 #define X86EMUL_CPUID_VENDOR_GenuineIntel_ecx 0x6c65746e
 #define X86EMUL_CPUID_VENDOR_GenuineIntel_edx 0x49656e69
 
+static inline bool is_guest_vendor_intel(u32 ebx, u32 ecx, u32 edx)
+{
+	return ebx == X86EMUL_CPUID_VENDOR_GenuineIntel_ebx &&
+	       ecx == X86EMUL_CPUID_VENDOR_GenuineIntel_ecx &&
+	       edx == X86EMUL_CPUID_VENDOR_GenuineIntel_edx;
+}
+
+static inline bool is_guest_vendor_amd(u32 ebx, u32 ecx, u32 edx)
+{
+	return (ebx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx &&
+		ecx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ecx &&
+		edx == X86EMUL_CPUID_VENDOR_AuthenticAMD_edx) ||
+	       (ebx == X86EMUL_CPUID_VENDOR_AMDisbetterI_ebx &&
+		ecx == X86EMUL_CPUID_VENDOR_AMDisbetterI_ecx &&
+		edx == X86EMUL_CPUID_VENDOR_AMDisbetterI_edx);
+}
+
+static inline bool is_guest_vendor_hygon(u32 ebx, u32 ecx, u32 edx)
+{
+	return ebx == X86EMUL_CPUID_VENDOR_HygonGenuine_ebx &&
+	       ecx == X86EMUL_CPUID_VENDOR_HygonGenuine_ecx &&
+	       edx == X86EMUL_CPUID_VENDOR_HygonGenuine_edx;
+}
+
 enum x86_intercept_stage {
 	X86_ICTP_NONE = 0,   /* Allow zero-init to not match anything */
 	X86_ICPT_PRE_EXCEPT,