[PATCH] KVM: arm64: Set a linux errno on SMCCC error in kvm_call_hyp_nvhe()

[Will Deacon]

On Wed, Jun 03, 2026 at 12:03:12PM +0100, Vincent Donnefort wrote:
> If the HVC called in kvm_call_hyp_nvhe() fails with an SMCCC error code,
> we WARN. However, the returned value isn't initialized and the caller
> might get garbage or 0 which is likely to be interpreted as success.
> 
> Set a default -EPERM error value, ensuring all callers get the message
> when SMCCC calls fail.
> 
> Signed-off-by: Vincent Donnefort <vdonnefort at google.com>
> 
> ---
> 
> I have encountered this issue while working on a follow-up contribution to the
> hypervisor tracing. In that case it completely crashed the kernel because
> IS_ERR() failed on that res.a1 value.
> 
> Now, if it makes that function more robust, I do not believe it is fixing any
> existing bug which is why I haven't added a "Fixes:" tag. 
> 
> In case we want to stick one, here it is:
> 
> Fixes: 054698316d87 ("KVM: arm64: nVHE: Migrate hyp interface to SMCCC")
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index a49042bfa801..6b8fd494792c 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -1273,13 +1273,14 @@ void kvm_arm_resume_guest(struct kvm *kvm);
>  #define vcpu_has_run_once(vcpu)	(!!READ_ONCE((vcpu)->pid))
>  
>  #ifndef __KVM_NVHE_HYPERVISOR__
> -#define kvm_call_hyp_nvhe(f, ...)						\
> +#define kvm_call_hyp_nvhe(f, ...)					\
>  	({								\
>  		struct arm_smccc_res res;				\
>  									\
>  		arm_smccc_1_1_hvc(KVM_HOST_SMCCC_FUNC(f),		\
>  				  ##__VA_ARGS__, &res);			\
> -		WARN_ON(res.a0 != SMCCC_RET_SUCCESS);			\
> +		if (WARN_ON(res.a0 != SMCCC_RET_SUCCESS))		\
> +			res.a1 = -EPERM;				\
>  									\
>  		res.a1;							\
>  	})

Looks like the only error code we return to the host is
SMCCC_RET_NOT_SUPPORTED, so maybe -EOPNOTSUPP would be more appropriate?

Either way:

Acked-by: Will Deacon <will at kernel.org>

Will