Re: [PATCH kvmtool v3 2/3] cpu: vmexit: Retry KVM_RUN ioctl on EINTR and EAGAIN

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

Hi Aneesh,

Is this to fix Will's report that the series breaks boot on x86?

On Mon, Apr 28, 2025 at 05:27:44PM +0530, Aneesh Kumar K.V (Arm) wrote:
> When KVM_RUN fails with EINTR or EAGAIN, we should retry the ioctl
> without checking kvm_run->exit_reason. These errors don't indicate a
> valid VM exit, hence exit_reason may contain stale or undefined values.

EAGAIN is not documented in Documentation/virt/kvm/api.rst. So I'm going to
assume it's this code path that is causing the -EAGAIN return value [1].

If that's the case, how does retrying KVM_RUN solve the issue? Just trying to
get to the bottom of it, because there's not much detail in the docs.

[1] https://elixir.bootlin.com/linux/v6.15-rc3/source/arch/x86/kvm/x86.c#L11532


Thanks,
Alex

> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  include/kvm/kvm-cpu.h |  2 +-
>  kvm-cpu.c             | 17 ++++++++++++-----
>  2 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/include/kvm/kvm-cpu.h b/include/kvm/kvm-cpu.h
> index 8f76f8a1123a..72cbb86e6cef 100644
> --- a/include/kvm/kvm-cpu.h
> +++ b/include/kvm/kvm-cpu.h
> @@ -16,7 +16,7 @@ void kvm_cpu__delete(struct kvm_cpu *vcpu);
>  void kvm_cpu__reset_vcpu(struct kvm_cpu *vcpu);
>  void kvm_cpu__setup_cpuid(struct kvm_cpu *vcpu);
>  void kvm_cpu__enable_singlestep(struct kvm_cpu *vcpu);
> -void kvm_cpu__run(struct kvm_cpu *vcpu);
> +int kvm_cpu__run(struct kvm_cpu *vcpu);
>  int kvm_cpu__start(struct kvm_cpu *cpu);
>  bool kvm_cpu__handle_exit(struct kvm_cpu *vcpu);
>  int kvm_cpu__get_endianness(struct kvm_cpu *vcpu);
> diff --git a/kvm-cpu.c b/kvm-cpu.c
> index 40041a22b3fe..7abbdcebf075 100644
> --- a/kvm-cpu.c
> +++ b/kvm-cpu.c
> @@ -35,27 +35,32 @@ void kvm_cpu__enable_singlestep(struct kvm_cpu *vcpu)
>  		pr_warning("KVM_SET_GUEST_DEBUG failed");
>  }
>  
> -void kvm_cpu__run(struct kvm_cpu *vcpu)
> +/*
> + * return value -1 if we need to call the kvm_cpu__run again without checking
> + * exit_reason. return value 0 results in taking action based on exit_reason.
> + */
> +int kvm_cpu__run(struct kvm_cpu *vcpu)
>  {
>  	int err;
>  
>  	if (!vcpu->is_running)
> -		return;
> +		return -1;
>  
>  	err = ioctl(vcpu->vcpu_fd, KVM_RUN, 0);
>  	if (err < 0) {
>  		switch (errno) {
>  		case EINTR:
>  		case EAGAIN:
> -			return;
> +			return -1;
>  		case EFAULT:
>  			if (vcpu->kvm_run->exit_reason == KVM_EXIT_MEMORY_FAULT)
> -				return;
> +				return 0;
>  			/* fallthrough */
>  		default:
>  			die_perror("KVM_RUN failed");
>  		}
>  	}
> +	return 0;
>  }
>  
>  static void kvm_cpu_signal_handler(int signum)
> @@ -179,7 +184,9 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
>  		if (cpu->task)
>  			kvm_cpu__run_task(cpu);
>  
> -		kvm_cpu__run(cpu);
> +		if (kvm_cpu__run(cpu) == -1)
> +			/* retry without an exit_reason check */
> +			continue;
>  
>  		switch (cpu->kvm_run->exit_reason) {
>  		case KVM_EXIT_UNKNOWN:
> -- 
> 2.43.0
> 
>