linux-kernel - [PATCH 3/9] KVM: SVM: Add a helper to detect VMRUN failures

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Message-ID: <20251113225621.1688428-4-seanjc@google.com>
Date: Thu, 13 Nov 2025 14:56:15 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>
Cc: kvm@...r.kernel.org, linux-hyperv@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [PATCH 3/9] 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@...gle.com>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/nested.c | 16 +++++++---------
 arch/x86/kvm/svm/svm.c    |  4 ++--
 arch/x86/kvm/svm/svm.h    |  5 +++++
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index ba0f11c68372..8070e20ed5a7 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1134,7 +1134,7 @@ 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))
@@ -1425,6 +1425,9 @@ 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);
@@ -1432,7 +1435,7 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
 	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,
@@ -1440,15 +1443,10 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
 		 */
 		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;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 7ea034ee6b6c..52b759408853 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3530,7 +3530,7 @@ 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;
@@ -4302,7 +4302,7 @@ 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;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 253a8dca412c..6b35925e3a33 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -426,6 +426,11 @@ 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.
-- 
2.52.0.rc1.455.g30608eb744-goog