KVM: SVM: Add a helper to detect VMRUN failures

Add a helper to detect VMRUN failures so that KVM can guard against its
own long-standing bug, where KVM neglects to set exitcode[63:32] when
synthesizing a nested VMFAIL_INVALID VM-Exit.  This will allow fixing
KVM's mess of treating exitcode as two separate 32-bit values without
breaking KVM-on-KVM when running on an older, unfixed KVM.

Cc: Jim Mattson <jmattson@google.com>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230211347.4099600-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/nested.c

@@ -1167,7 +1167,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 	vmcb12->control.exit_info_1       = vmcb02->control.exit_info_1;
 	vmcb12->control.exit_info_2       = vmcb02->control.exit_info_2;
 
-	if (vmcb12->control.exit_code != SVM_EXIT_ERR)
+	if (!svm_is_vmrun_failure(vmcb12->control.exit_code))
 		nested_save_pending_event_to_vmcb12(svm, vmcb12);
 
 	if (guest_cpu_cap_has(vcpu, X86_FEATURE_NRIPS))
@@ -1463,30 +1463,28 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
 	u32 exit_code = svm->vmcb->control.exit_code;
 	int vmexit = NESTED_EXIT_HOST;
 
+	if (svm_is_vmrun_failure(exit_code))
+		return NESTED_EXIT_DONE;
+
 	switch (exit_code) {
 	case SVM_EXIT_MSR:
 		vmexit = nested_svm_exit_handled_msr(svm);
 		break;
 	case SVM_EXIT_IOIO:
 		vmexit = nested_svm_intercept_ioio(svm);
 		break;
-	case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f: {
+	case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f:
 		/*
 		 * Host-intercepted exceptions have been checked already in
 		 * nested_svm_exit_special.  There is nothing to do here,
 		 * the vmexit is injected by svm_check_nested_events.
 		 */
 		vmexit = NESTED_EXIT_DONE;
 		break;
-	}
-	case SVM_EXIT_ERR: {
-		vmexit = NESTED_EXIT_DONE;
-		break;
-	}
-	default: {
+	default:
 		if (vmcb12_is_intercept(&svm->nested.ctl, exit_code))
 			vmexit = NESTED_EXIT_DONE;
-	}
+		break;
 	}
 
 	return vmexit;

arch/x86/kvm/svm/svm.c

@@ -3564,7 +3564,7 @@ static int svm_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
 			return 1;
 	}
 
-	if (svm->vmcb->control.exit_code == SVM_EXIT_ERR) {
+	if (svm_is_vmrun_failure(svm->vmcb->control.exit_code)) {
 		kvm_run->exit_reason = KVM_EXIT_FAIL_ENTRY;
 		kvm_run->fail_entry.hardware_entry_failure_reason
 			= svm->vmcb->control.exit_code;
@@ -4346,7 +4346,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
 
 		/* Track VMRUNs that have made past consistency checking */
 		if (svm->nested.nested_run_pending &&
-		    svm->vmcb->control.exit_code != SVM_EXIT_ERR)
+		    !svm_is_vmrun_failure(svm->vmcb->control.exit_code))
                         ++vcpu->stat.nested_run;
 
 		svm->nested.nested_run_pending = 0;

arch/x86/kvm/svm/svm.h

@@ -425,6 +425,11 @@ static __always_inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
 	return container_of(vcpu, struct vcpu_svm, vcpu);
 }
 
+static inline bool svm_is_vmrun_failure(u64 exit_code)
+{
+	return (u32)exit_code == (u32)SVM_EXIT_ERR;
+}
+
 /*
  * Only the PDPTRs are loaded on demand into the shadow MMU.  All other
  * fields are synchronized on VM-Exit, because accessing the VMCB is cheap.