KVM: x86: Allow SYSENTER in Compatibility Mode for all Intel compat vCPUs

Emulate SYSENTER in Compatibility Mode for all vCPUs models that are
compatible with Intel's architecture, as the behavior if SYSENTER is
architecturally defined in Intel's SDM, i.e. should be followed by any
CPU that implements Intel's architecture.

Link: https://lore.kernel.org/r/20240405235603.1173076-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/emulate.c

@@ -2354,6 +2354,7 @@ setup_syscalls_segments(struct desc_struct *cs, struct desc_struct *ss)
 	ss->avl = 0;
 }
 
+#ifdef CONFIG_X86_64
 static bool vendor_intel(struct x86_emulate_ctxt *ctxt)
 {
 	u32 eax, ebx, ecx, edx;
@@ -2362,6 +2363,7 @@ static bool vendor_intel(struct x86_emulate_ctxt *ctxt)
 	ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx, true);
 	return is_guest_vendor_intel(ebx, ecx, edx);
 }
+#endif
 
 static int em_syscall(struct x86_emulate_ctxt *ctxt)
 {
@@ -2444,11 +2446,11 @@ static int em_sysenter(struct x86_emulate_ctxt *ctxt)
 		return emulate_gp(ctxt, 0);
 
 	/*
-	 * Not recognized on AMD in compat mode (but is recognized in legacy
-	 * mode).
+	 * Intel's architecture allows SYSENTER in compatibility mode, but AMD
+	 * does not.  Note, AMD does allow SYSENTER in legacy protected mode.
 	 */
-	if ((ctxt->mode != X86EMUL_MODE_PROT64) && (efer & EFER_LMA)
-	    && !vendor_intel(ctxt))
+	if ((ctxt->mode != X86EMUL_MODE_PROT64) && (efer & EFER_LMA) &&
+	    !ctxt->ops->guest_cpuid_is_intel_compatible(ctxt))
 		return emulate_ud(ctxt);
 
 	/* sysenter/sysexit have not been tested in 64bit mode. */