KVM: x86: Use "is Intel compatible" helper to emulate SYSCALL in !64-bit

Use guest_cpuid_is_intel_compatible() to determine whether SYSCALL in
32-bit Protected Mode (including Compatibility Mode) should #UD or succeed.
The existing code already does the exact equivalent of
guest_cpuid_is_intel_compatible(), just in a rather roundabout way.

No functional change intended.

Link: https://lore.kernel.org/r/20240405235603.1173076-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/emulate.c

@@ -2363,41 +2363,6 @@ static bool vendor_intel(struct x86_emulate_ctxt *ctxt)
 	return is_guest_vendor_intel(ebx, ecx, edx);
 }
 
-static bool em_syscall_is_enabled(struct x86_emulate_ctxt *ctxt)
-{
-	const struct x86_emulate_ops *ops = ctxt->ops;
-	u32 eax, ebx, ecx, edx;
-
-	/*
-	 * syscall should always be enabled in longmode - so only become
-	 * vendor specific (cpuid) if other modes are active...
-	 */
-	if (ctxt->mode == X86EMUL_MODE_PROT64)
-		return true;
-
-	eax = 0x00000000;
-	ecx = 0x00000000;
-	ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx, true);
-	/*
-	 * 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 (is_guest_vendor_intel(ebx, ecx, edx))
-		return false;
-
-	if (is_guest_vendor_amd(ebx, ecx, edx) ||
-	    is_guest_vendor_hygon(ebx, ecx, edx))
-		return true;
-
-	/*
-	 * default: (not Intel, not AMD, not Hygon), apply Intel's
-	 * stricter rules...
-	 */
-	return false;
-}
-
 static int em_syscall(struct x86_emulate_ctxt *ctxt)
 {
 	const struct x86_emulate_ops *ops = ctxt->ops;
@@ -2411,7 +2376,15 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
 	    ctxt->mode == X86EMUL_MODE_VM86)
 		return emulate_ud(ctxt);
 
-	if (!(em_syscall_is_enabled(ctxt)))
+	/*
+	 * Intel compatible CPUs only support SYSCALL in 64-bit mode, whereas
+	 * AMD allows SYSCALL in any flavor of protected mode.  Note, it's
+	 * infeasible to emulate Intel behavior when running on AMD hardware,
+	 * as SYSCALL won't fault in the "wrong" mode, i.e. there is no #UD
+	 * for KVM to trap-and-emulate, unlike emulating AMD on Intel.
+	 */
+	if (ctxt->mode != X86EMUL_MODE_PROT64 &&
+	    ctxt->ops->guest_cpuid_is_intel_compatible(ctxt))
 		return emulate_ud(ctxt);
 
 	ops->get_msr(ctxt, MSR_EFER, &efer);

arch/x86/kvm/kvm_emulate.h

@@ -223,6 +223,7 @@ struct x86_emulate_ops {
 	bool (*guest_has_movbe)(struct x86_emulate_ctxt *ctxt);
 	bool (*guest_has_fxsr)(struct x86_emulate_ctxt *ctxt);
 	bool (*guest_has_rdpid)(struct x86_emulate_ctxt *ctxt);
+	bool (*guest_cpuid_is_intel_compatible)(struct x86_emulate_ctxt *ctxt);
 
 	void (*set_nmi_mask)(struct x86_emulate_ctxt *ctxt, bool masked);
 

arch/x86/kvm/x86.c

@@ -8549,6 +8549,11 @@ static bool emulator_guest_has_rdpid(struct x86_emulate_ctxt *ctxt)
 	return guest_cpuid_has(emul_to_vcpu(ctxt), X86_FEATURE_RDPID);
 }
 
+static bool emulator_guest_cpuid_is_intel_compatible(struct x86_emulate_ctxt *ctxt)
+{
+	return guest_cpuid_is_intel_compatible(emul_to_vcpu(ctxt));
+}
+
 static ulong emulator_read_gpr(struct x86_emulate_ctxt *ctxt, unsigned reg)
 {
 	return kvm_register_read_raw(emul_to_vcpu(ctxt), reg);
@@ -8647,6 +8652,7 @@ static const struct x86_emulate_ops emulate_ops = {
 	.guest_has_movbe     = emulator_guest_has_movbe,
 	.guest_has_fxsr      = emulator_guest_has_fxsr,
 	.guest_has_rdpid     = emulator_guest_has_rdpid,
+	.guest_cpuid_is_intel_compatible = emulator_guest_cpuid_is_intel_compatible,
 	.set_nmi_mask        = emulator_set_nmi_mask,
 	.is_smm              = emulator_is_smm,
 	.is_guest_mode       = emulator_is_guest_mode,