Re: [PATCH kvmtool 2/2] cpu: vmexit: Handle KVM_EXIT_MEMORY_FAULT correctly

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Tue, Feb 11, 2025 at 01:08:52PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Linux kernel documentation states:
> 
> "Note! KVM_EXIT_MEMORY_FAULT is unique among all KVM exit reasons in
> that it accompanies a return code of '-1', not '0'! errno will always be
> set to EFAULT or EHWPOISON when KVM exits with KVM_EXIT_MEMORY_FAULT,
> userspace should assume kvm_run.exit_reason is stale/undefined for all
> other error numbers." "
> 
> Update the KVM_RUN ioctl error handling to correctly handle
> KVM_EXIT_MEMORY_FAULT.

I've tried to follow how kvmtool handles KVM_EXIT_MEMORY_FAULT before and after
this patch.

Before: calls die_perror().
After: prints more information about the error, in kvm_cpu_thread().

Is that what you want? Because "correctly handle KVM_EXIT_MEMORY_FAULT" can be
interpreted as kvmtool resolving the memory fault, which is something that
kvmtool does not do.

Also, can you update kvm_exit_reasons with KVM_EXIT_MEMORY_FAULT, because
otherwise kvm_cpu_thread() will segfault when it tries to access
kvm_exit_reasons[KVM_EXIT_MEMORY_FAULT].

Thanks,
Alex

> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  kvm-cpu.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kvm-cpu.c b/kvm-cpu.c
> index 66e30ba54e26..40e4efc33a1d 100644
> --- a/kvm-cpu.c
> +++ b/kvm-cpu.c
> @@ -41,8 +41,15 @@ void kvm_cpu__run(struct kvm_cpu *vcpu)
>  		return;
>  
>  	err = ioctl(vcpu->vcpu_fd, KVM_RUN, 0);
> -	if (err < 0 && (errno != EINTR && errno != EAGAIN))
> -		die_perror("KVM_RUN failed");
> +	if (err < 0) {
> +		if (errno == EINTR || errno == EAGAIN)
> +			return;
> +		else if (errno == EFAULT &&
> +			 vcpu->kvm_run->exit_reason == KVM_EXIT_MEMORY_FAULT)
> +			return;
> +		else
> +			die_perror("KVM_RUN failed");
> +	}
>  }
>  
>  static void kvm_cpu_signal_handler(int signum)
> -- 
> 2.43.0
> 
>