Re: [PATCH v2 2/6] KVM: nSVM: Simplify error handling of nested_svm_copy_vmcb12_to_cache()

[Sean Christopherson]

On Fri, Mar 06, 2026, Yosry Ahmed wrote:
> nested_svm_vmrun() currently stores the return value of
> nested_svm_copy_vmcb12_to_cache() in a local variable 'err', separate
> from the generally used 'ret' variable. This is done to have a single
> call to kvm_skip_emulated_instruction(), such that we can store the
> return value of kvm_skip_emulated_instruction() in 'ret', and then
> re-check the return value of nested_svm_copy_vmcb12_to_cache() in 'err'.
> 
> The code is unnecessarily confusing. Instead, call
> kvm_skip_emulated_instruction() in the failure path of
> nested_svm_copy_vmcb12_to_cache() if the return value is not -EFAULT,
> and drop 'err'.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>

FYI, I'm going to grab this right now to make it slightly easier to resolve the
merge conflict with Paolo's SMM fixes (the ret vs. err stuff is so confusing).

> ---
>  arch/x86/kvm/svm/nested.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index b191c6cab57db..6d4c053778b21 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1079,7 +1079,7 @@ static int nested_svm_copy_vmcb12_to_cache(struct kvm_vcpu *vcpu, u64 vmcb12_gpa
>  int nested_svm_vmrun(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> -	int ret, err;
> +	int ret;
>  	u64 vmcb12_gpa;
>  	struct vmcb *vmcb01 = svm->vmcb01.ptr;
>  
> @@ -1104,19 +1104,20 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
>  		return -EINVAL;
>  
>  	vmcb12_gpa = svm->vmcb->save.rax;
> -	err = nested_svm_copy_vmcb12_to_cache(vcpu, vmcb12_gpa);
> -	if (err == -EFAULT) {
> +	ret = nested_svm_copy_vmcb12_to_cache(vcpu, vmcb12_gpa);
> +
> +	/*
> +	 * Advance RIP if #GP or #UD are not injected, but otherwise
> +	 * stop if copying and checking vmcb12 failed.
> +	 */
> +	if (ret == -EFAULT) {
>  		kvm_inject_gp(vcpu, 0);
>  		return 1;
> +	} else if (ret) {
> +		return kvm_skip_emulated_instruction(vcpu);
>  	}

I strongly dislike the if-elif approach, because it makes unnecessarily hard to
see that *all* ret !=0 cases are handled, i.e. that overwriting ret below is ok.

The comment is also super confusing, because there's no #UD in sight, but there
is a #GP. 

This is what I have locally and am planning on pushing to kvm-x86/next.

	ret = nested_svm_copy_vmcb12_to_cache(vcpu, vmcb12_gpa);
	if (ret) {
		if (ret == -EFAULT) {
			kvm_inject_gp(vcpu, 0);
			return 1;
		}

		/* Advance RIP past VMRUN as part of the nested #VMEXIT. */
		return kvm_skip_emulated_instruction(vcpu);
	}

	/* At this point, VMRUN is guaranteed to not fault; advance RIP. */
	ret = kvm_skip_emulated_instruction(vcpu);

>  
> -	/*
> -	 * Advance RIP if #GP or #UD are not injected, but otherwise stop if
> -	 * copying and checking vmcb12 failed.
> -	 */
>  	ret = kvm_skip_emulated_instruction(vcpu);
> -	if (err)
> -		return ret;
>  
>  	/*
>  	 * Since vmcb01 is not in use, we can use it to store some of the L1
> -- 
> 2.53.0.473.g4a7958ca14-goog
>