Re: [PATCH kvmtool 1/2] cpu: vmexit: Handle KVM_EXIT_UNKNOWN exit reason correctly

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

Hi,

On Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote:
> The return value for the KVM_RUN ioctl is confusing and has led to
> errors in different kernel exit handlers. A return value of 0 indicates
> a return to the VMM, whereas a return value of 1 indicates resuming
> execution in the guest. Some handlers mistakenly return 0 to force a
> return to the guest.

I find this paragraph confusing. KVM_RUN, as per the documentation, returns 0 or
-1 (on error). As far as I can tell, at least on arm64, KVM_RUN can never return
1.

Are you referring to the loop in kvm_arch_vcpu_ioctl_run() by any chance? That's
the only place I found where a value of 1 from the handlers signifies return to
the guest.

> 
> This worked in kvmtool because the exit_reason defaulted to
> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit
> reason. However, forcing a KVM panic on an unknown exit reason would
> help catch these bugs early.

I would hope that a VMM cannot force KVM to panic at will, which will bring down
the host. Are you referring to kvmtool exiting with an error? That's what the
unfortunately named 'panic_kvm' label seems to be doing.

Thanks,
Alex

> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>  kvm-cpu.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kvm-cpu.c b/kvm-cpu.c
> index f66dcd07220c..66e30ba54e26 100644
> --- a/kvm-cpu.c
> +++ b/kvm-cpu.c
> @@ -170,6 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
>  
>  		switch (cpu->kvm_run->exit_reason) {
>  		case KVM_EXIT_UNKNOWN:
> +			goto panic_kvm;
>  			break;
>  		case KVM_EXIT_DEBUG:
>  			kvm_cpu__show_registers(cpu);
> 
> base-commit: 6d754d01fe2cb366f3b636d8a530f46ccf3b10d8
> -- 
> 2.43.0
> 
>