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

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Message-ID: <nwvfxozjx6l5yaflvlbbnxqu3opagtnifmmsdv6y3aco3tyhdd@rooicluxzmiu>
Date: Thu, 13 Nov 2025 23:17:22 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: 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>, 
	kvm@...r.kernel.org, linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 2/9] KVM: nSVM: Set exit_code_hi to -1 when synthesizing
 SVM_EXIT_ERR (failed VMRUN)

On Thu, Nov 13, 2025 at 02:56:14PM -0800, Sean Christopherson wrote:
> 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>

Reviewed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>

> ---
>  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
>