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

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

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

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

Yes.

> 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].
>

IIUC, EAGAIN and EINTR are syscall return errno that indicates a system
call need to be retried. Documentation/virt/kvm/api.rst do mention that
exit_reason is value only with return value 0.

	__u32 exit_reason;

When KVM_RUN has returned successfully (return value 0), this informs
application code why KVM_RUN has returned.

>
> 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
>
>

So in that code path vcpu will be in kvm_vcpu_block(vcpu) waiting for
the IPI. On IPI kvm_apic_accept_events() will return 0 after setting the
vcpu->arch.mp_state. Hence a KVM_RUN ioctl again will find the mp_state
correctly updated. 

-aneesh