Re: xen/evtchn and forced threaded irq

[Julien Grall <julien.grall@arm.com>]

Hi,

On 2/26/19 11:02 AM, Roger Pau Monné wrote:
> On Tue, Feb 26, 2019 at 10:26:21AM +0000, Julien Grall wrote:
>> On 26/02/2019 10:17, Roger Pau Monné wrote:
>>> On Tue, Feb 26, 2019 at 10:03:38AM +0000, Julien Grall wrote:
>>>> Hi Roger,
>>>>
>>>> On 26/02/2019 09:44, Roger Pau Monné wrote:
>>>>> On Tue, Feb 26, 2019 at 09:30:07AM +0000, Andrew Cooper wrote:
>>>>>> On 26/02/2019 09:14, Roger Pau Monné wrote:
>>>>>>> On Mon, Feb 25, 2019 at 01:55:42PM +0000, Julien Grall wrote:
>>>>>>>> Hi Oleksandr,
>>>>>>>>
>>>>>>>> On 25/02/2019 13:24, Oleksandr Andrushchenko wrote:
>>>>>>>>> On 2/22/19 3:33 PM, Julien Grall wrote:
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> On 22/02/2019 12:38, Oleksandr Andrushchenko wrote:
>>>>>>>>>>> On 2/20/19 10:46 PM, Julien Grall wrote:
>>>>>>>>>>>> Discussing with my team, a solution that came up would be to
>>>>>>>>>>>> introduce one atomic field per event to record the number of
>>>>>>>>>>>> event received. I will explore that solution tomorrow.
>>>>>>>>>>> How will this help if events have some payload?
>>>>>>>>>> What payload? The event channel does not carry any payload. It only
>>>>>>>>>> notify you that something happen. Then this is up to the user to
>>>>>>>>>> decide what to you with it.
>>>>>>>>> Sorry, I was probably not precise enough. I mean that an event might have
>>>>>>>>> associated payload in the ring buffer, for example [1]. So, counting events
>>>>>>>>> may help somehow, but the ring's data may still be lost
>>>>>>>>    From my understanding of event channels are edge interrupts. By definition,
>>>>>>> IMO event channels are active high level interrupts.
>>>>>>>
>>>>>>> Let's take into account the following situation: you have an event
>>>>>>> channel masked and the event channel pending bit (akin to the line on
>>>>>>> bare metal) goes from low to high (0 -> 1), then you unmask the
>>>>>>> interrupt and you get an event injected. If it was an edge interrupt
>>>>>>> you wont get an event injected after unmasking, because you would
>>>>>>> have lost the edge. I think the problem here is that Linux treats
>>>>>>> event channels as edge interrupts, when they are actually level.
>>>>>>
>>>>>> Event channels are edge interrupts.  There are several very subtle bugs
>>>>>> to be had by software which treats them as line interrupts.
>>>>>>
>>>>>> Most critically, if you fail to ack them, rebind them to a new vcpu, and
>>>>>> reenable interrupts, you don't get a new interrupt notification.  This
>>>>>> was the source of a 4 month bug when XenServer was moving from
>>>>>> classic-xen to PVOps where using irqbalance would cause dom0 to
>>>>>> occasionally lose interrupts.
>>>>>
>>>>> I would argue that you need to mask them first, rebind to a new vcpu
>>>>> and unmask, and then you will get an interrupt notification, or this
>>>>> should be fixed in Xen to work as you expect: trigger an interrupt
>>>>> notification when moving an asserted event channel between CPUs.
>>>>>
>>>>> Is there any document that describes how such non trivial things (like
>>>>> moving between CPUs) work for event/level interrupts?
>>>>>
>>>>> Maybe I'm being obtuse, but from the example I gave above it's quite
>>>>> clear to me event channels don't get triggered based on edge changes,
>>>>> but rather on the line level.
>>>>
>>>> Your example above is not enough to give the semantics of level. You would
>>>> only use the MASK bit if your interrupt handler is threaded to avoid the
>>>> interrupt coming up again.
>>>>
>>>> So if you remove the mask from the equation, then the interrupt flow should be:
>>>>
>>>> 1) handle interrupt
>>>> 2) EOI
>>>
>>> This is bogus if you don't mask the interrupt source. You should
>>> instead do
>>>
>>> 1) EOI
>>> 2) Handle interrupt
>>>
>>> And loop over this.
>> So that's not a level semantics. It is a edge one :). In the level case, you
>> would clear the state once you are done with the interrupt.
>>
>> Also, it would be ACK and not EOI.
> 
> For level triggered interrupts you have to somehow signal the device
> to stop asserting the line, which doesn't happen for Xen devices
> because they just signal interrupts to Xen, but don't have a way to
> keep event channels asserted, so I agree that this is different from
> traditional level interrupts because devices using event channels
> don't have a way to keep lines asserted.
> 
> I guess the most similar native interrupt is MSI with masking
> support?

I don't know enough about MSI with masking support to be able to draw a 
comparison :).

The flow I have been suggested to re-use in Linux is 
handle_fasteoi_ack_irq. I haven't yet had time to have a try at it.

Cheers,

-- 
Julien Grall

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