linux-kernel - Re: [PATCH 8/9] KVM: SVM: Limit incorrect check on SVM_EXIT

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Message-ID: <6n66ud7xoqii4zlbitdnizx6tclcv4fzglqi4lbb2tgi3vp7lh@6bef4riuvyod>
Date: Fri, 14 Nov 2025 00:11:33 +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 8/9] KVM: SVM: Limit incorrect check on SVM_EXIT_ERR to
 running as a VM

On Thu, Nov 13, 2025 at 02:56:20PM -0800, Sean Christopherson wrote:
> Limit KVM's incorrect check for VMXEXIT_INVALID, a.k.a. SVM_EXIT_ERR, to
> running as a VM, as detected by X86_FEATURE_HYPERVISOR.  The exit_code and
> all failure codes, e.g. VMXEXIT_INVALID, are 64-bit values, and so checking
> only bits 31:0 could result in false positives when running on non-broken
> hardware, e.g. in the extremely unlikely scenario exit code 0xffffffffull
> is ever generated by hardware.
> 
> Keep the 32-bit check to play nice with running on broken KVM (for years,
> KVM has not set bits 63:32 when synthesizing nested SVM VM-Exits).
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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

> ---
>  arch/x86/kvm/svm/svm.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 31ee4f65dcc2..801dcfc64d0b 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -427,7 +427,10 @@ static __always_inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
>  
>  static inline bool svm_is_vmrun_failure(u64 exit_code)
>  {
> -	return (u32)exit_code == (u32)SVM_EXIT_ERR;
> +	if (cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
> +		return (u32)exit_code == (u32)SVM_EXIT_ERR;
> +
> +	return exit_code == SVM_EXIT_ERR;
>  }
>  
>  /*
> -- 
> 2.52.0.rc1.455.g30608eb744-goog
>