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

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Message-ID: <ellmjkhqmgpsbhc4if3emhn3fzbqd3ji4u2dnyvmub6bjgfnti@vtvjhn5cjwrs>
Date: Thu, 13 Nov 2025 23:30:50 +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 3/9] KVM: SVM: Add a helper to detect VMRUN failures

On Thu, Nov 13, 2025 at 02:56:15PM -0800, Sean Christopherson wrote:
> 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))

This was flipped, wasn't it?

>  		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
>