Re: [PATCH 2/2] xen: make xen_qlock_wait() nestable

[Juergen Gross]

On 10/10/2018 14:47, Thomas Gleixner wrote:
> On Wed, 10 Oct 2018, David Woodhouse wrote:
>> On Wed, 2018-10-10 at 14:30 +0200, Thomas Gleixner wrote:
>>> On Wed, 10 Oct 2018, David Woodhouse wrote:
>>>
>>>> On Mon, 2018-10-01 at 09:16 +0200, Juergen Gross wrote:
>>>>> -       /* If irq pending already clear it and return. */
>>>>> +       /* Guard against reentry. */
>>>>> +       local_irq_save(flags);
>>>>> +
>>>>> +       /* If irq pending already clear it. */
>>>>>         if (xen_test_irq_pending(irq)) {
>>>>>                 xen_clear_irq_pending(irq);
>>>>> -               return;
>>>>> +       } else if (READ_ONCE(*byte) == val) {
>>>>> +               /* Block until irq becomes pending (or a spurious wakeup) */
>>>>> +               xen_poll_irq(irq);
>>>>>         }
>>>>
>>>>
>>>> Does this still allow other IRQs to wake it from xen_poll_irq()?
>>>>
>>>> In the case where process-context code is spinning for a lock without
>>>> disabling interrupts, we *should* allow interrupts to occur still...
>>>> does this?
>>>
>>> Yes. Look at it like idle HLT or WFI. You have to disable interrupt before
>>> checking the condition and then the hardware or in this case the hypervisor
>>> has to bring you back when an interrupt is raised.
>>>
>>> If that would not work then the check would be racy, because the interrupt
>>> could hit and be handled after the check and before going into
>>> HLT/WFI/hypercall and then the thing is out until the next interrupt comes
>>> along, which might be never.
>>
>> Right, but in this case we're calling into the hypervisor to poll for
>> one *specific* IRQ. Everything you say is true for that specific IRQ.
>>
>> My question is what happens to *other* IRQs. We want them, but are they
>> masked? I'm staring at the Xen do_poll() code and haven't quite worked
>> that out...
> 
> Ah, sorry. That of course has to come back like HLT/WFI for any interrupt,
> but I have no idea what the Xen HV is doing there.

The Xen HV is doing it right. It is blocking the vcpu in do_poll() and
any interrupt will unblock it.


Juergen