Re: [PATCH 1/2] KVM: x86/xen: Fix return code when clearing vcpu_info and vcpu_time_info

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 01, 2021, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> When clearing the per-vCPU shared regions, set the return value to zero
> to indicate success. This was causing spurious errors to be returned to
> userspace on soft reset.
> 
> Also add a paranoid BUILD_BUG_ON() for compat structure compatibility.
> 
> Fixes: 0c165b3c01fe ("KVM: x86/xen: Allow reset of Xen attributes")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  arch/x86/kvm/xen.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index af8f6562fce4..77b20ff09078 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -187,9 +187,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)

To avoid similar issues in the future, might be worth throwing "r = -ENOENT" in
the default case and have "r" be initialized to '0', or uninitialized to trigger
compiler warnings.  For any which way:

Reviewed-by: Sean Christopherson <seanjc@google.com>

>  		/* No compat necessary here. */
>  		BUILD_BUG_ON(sizeof(struct vcpu_info) !=
>  			     sizeof(struct compat_vcpu_info));
> +		BUILD_BUG_ON(offsetof(struct vcpu_info, time) !=
> +			     offsetof(struct compat_vcpu_info, time));
>  
>  		if (data->u.gpa == GPA_INVALID) {
>  			vcpu->arch.xen.vcpu_info_set = false;
> +			r = 0;
>  			break;
>  		}
>  
> @@ -206,6 +209,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  	case KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO:
>  		if (data->u.gpa == GPA_INVALID) {
>  			vcpu->arch.xen.vcpu_time_info_set = false;
> +			r = 0;
>  			break;
>  		}
>  
> -- 
> 2.29.2
>