linux-kernel - [PATCH 2/9] KVM: nSVM: Set exit_code_hi to -1 when synthesizing SVM_EXIT

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Message-ID: <20251113225621.1688428-3-seanjc@google.com>
Date: Thu, 13 Nov 2025 14:56:14 -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 2/9] KVM: nSVM: Set exit_code_hi to -1 when synthesizing
 SVM_EXIT_ERR (failed VMRUN)

Set exit_code_hi to -1u as a temporary band-aid to fix a long-standing
(effectively since KVM's inception) bug where KVM treats the exit code as
a 32-bit value, when in reality it's a 64-bit value.  Per the APM, offset
0x70 is a single 64-bit value:

  070h 63:0 EXITCODE

And a sane reading of the error values defined in "Table C-1. SVM Intercept
Codes" is that negative values use the full 64 bits:

  –1 VMEXIT_INVALID Invalid guest state in VMCB.
  –2 VMEXIT_BUSYBUSY bit was set in the VMSA
  –3 VMEXIT_IDLE_REQUIREDThe sibling thread is not in an idle state
  -4 VMEXIT_INVALID_PMC Invalid PMC state

And that interpretation is confirmed by testing on Milan and Turin (by
setting bits in CR0[63:32] to generate VMEXIT_INVALID on VMRUN).

Furthermore, Xen has treated exitcode as a 64-bit value since HVM support
was adding in 2006 (see Xen commit d1bd157fbc ("Big merge the HVM
full-virtualisation abstractions.")).

Cc: Jim Mattson <jmattson@...gle.com>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/nested.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index c81005b24522..ba0f11c68372 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -985,7 +985,7 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
 	if (!nested_vmcb_check_save(vcpu) ||
 	    !nested_vmcb_check_controls(vcpu)) {
 		vmcb12->control.exit_code    = SVM_EXIT_ERR;
-		vmcb12->control.exit_code_hi = 0;
+		vmcb12->control.exit_code_hi = -1u;
 		vmcb12->control.exit_info_1  = 0;
 		vmcb12->control.exit_info_2  = 0;
 		goto out;
@@ -1018,7 +1018,7 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
 	svm->soft_int_injected = false;
 
 	svm->vmcb->control.exit_code    = SVM_EXIT_ERR;
-	svm->vmcb->control.exit_code_hi = 0;
+	svm->vmcb->control.exit_code_hi = -1u;
 	svm->vmcb->control.exit_info_1  = 0;
 	svm->vmcb->control.exit_info_2  = 0;
 
-- 
2.52.0.rc1.455.g30608eb744-goog