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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Alexandru Elisei <alexandru.elisei@arm.com> writes:

> 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.
>

Yes. I will update the commit message to reflect that. It is the exit
handler return value rather than KVM_RUN ioctl return value.

>
>> 
>> 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.
>

yes. I will update the commit message to indicate that for
KVM_EXIT_UNKNOWN exit reason, kvmtool will now exit with an error
instead of returning to the guest."

-aneesh