KVM: selftests: Add vmgexit helper

Abstract rep vmmcall coded into the vmgexit helper for the sev
library.

No functional change intended.

Signed-off-by: Pratik R. Sampat <prsampat@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://lore.kernel.org/r/20250305230000.231025-4-prsampat@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/x86/sev.h

@@ -71,6 +71,11 @@ kvm_static_assert(SEV_RET_SUCCESS == 0);
 void sev_vm_init(struct kvm_vm *vm);
 void sev_es_vm_init(struct kvm_vm *vm);
 
+static inline void vmgexit(void)
+{
+	__asm__ __volatile__("rep; vmmcall");
+}
+
 static inline void sev_register_encrypted_memory(struct kvm_vm *vm,
 						 struct userspace_mem_region *region)
 {

tools/testing/selftests/kvm/x86/sev_smoke_test.c

@@ -27,7 +27,7 @@ static void guest_sev_es_code(void)
 	 * force "termination" to signal "done" via the GHCB MSR protocol.
 	 */
 	wrmsr(MSR_AMD64_SEV_ES_GHCB, GHCB_MSR_TERM_REQ);
-	__asm__ __volatile__("rep; vmmcall");
+	vmgexit();
 }
 
 static void guest_sev_code(void)