Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

Julien Grall
2021-02-08T12:19:20Z

On 08/02/2021 12:14, Jürgen Groß wrote:
> On 08.02.21 11:40, Julien Grall wrote:
>> Hi Juergen,
>>
>> On 08/02/2021 10:22, Jürgen Groß wrote:
>>> On 08.02.21 10:54, Julien Grall wrote:
>>>> ... I don't really see how the difference matter here. The idea is
>>>> to re-use what's already existing rather than trying to re-invent
>>>> the wheel with an extra lock (or whatever we can come up).
>>>
>>> The difference is that the race is occurring _before_ any IRQ is
>>> involved. So I don't see how modification of IRQ handling would help.
>>
>> Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:
>>
>> if ( irq in progress )
>> {
>>    set IRQS_PENDING
>>    return;
>> }
>>
>> do
>> {
>>    clear IRQS_PENDING
>>    handle_irq()
>> } while (IRQS_PENDING is set)
>>
>> IRQ handling flow like handle_fasteoi_irq() looks like:
>>
>> if ( irq in progress )
>>    return;
>>
>> handle_irq()
>>
>> The latter flow would catch "spurious" interrupt and ignore them. So
>> it would handle nicely the race when changing the event affinity.
>
> Sure? Isn't "irq in progress" being reset way before our "lateeoi" is
> issued, thus having the same problem again?

Sorry I can't parse this.

And I think we want to keep
> the lateeoi behavior in order to be able to control event storms.

I didn't (yet) suggest to remove lateeoi. I only suggest to use a
different workflow to handle the race with vCPU affinity.

Cheers,

--
Julien Grall