Re: [PATCH 4/8] i386: hvf: Implement CPU kick

[Paolo Bonzini]

On 25/06/20 17:57, Roman Bolshakov wrote:
> So, the kick is not delivered to self and in case if destination cpu is
> not running. I think it can't interrupt subsequent hv_vcpu_run.

Yes.

>> If not, you can reduce a bit the race window by setting a variable in
>> cpu, like
>>
>>      atomic_set(&cpu->deadline, 0);
>>      hv_vcpu_interrupt(...)
>>
>> and in the vCPU thread
>>
>>      hv_vcpu_run_until(..., atomic_read(&cpu->deadline));
>>      atomic_set(&cpu->deadline, HV_DEADLINE_FOREVER);
>>
> 
> Sure, could you please explain who'll be racing? There's a race if a
> kick was sent after VMEXIT, right? So essentially we need a way to
> "requeue" a kick that was received outside of hv_vcpu_run to avoid loss
> of it?

Yes.  Note that this is not a new bug, it's pre-existing and it's common
to all hypervisors except KVM/WHPX.  I mean not the QEMU code, it's the
kernel APIs that are broken. :)

One way to do so is to keep the signal, and have the signal handler
enable the preemption timer (with a deadline of 0) in the pin-based
interrupt controls.  Hopefully macOS allows that, especially on 10.15+
where hv_vcpu_run_until probably uses the preemption timer.

> hv_vcpu_run_until is only available on macOS 10.15+ and we can't use yet
> because of three release support rule.
> (https://developer.apple.com/documentation/hypervisor/3181548-hv_vcpu_run_until?language=objc)
> 
> BTW, I'm totally okay to send v2 if kicks are lost and/or the patch
> needs improvements. (and I can address EFER to VMCS Entry Controls
> synchronization as well)
> 
> Paolo, do you know any particular test in kvm-unit-tests that can
> exhibit the issue?

No, it's a race and it's extremely rare, but I point it out because it's
a kernel issue that Apple might want to fix anyway.  It might also be
(depending on how the kernel side is written) that the next scheduler
tick will end up unblocking the vCPU and papering over it.

Paolo