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Subject: Re: [Xen-devel] xen/evtchn and forced threaded irq
To:     =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
Cc:     Andrew Cooper <andrew.cooper3@citrix.com>,
        Oleksandr Andrushchenko <andr2000@gmail.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jan Beulich <JBeulich@suse.com>,
        xen-devel <xen-devel@lists.xenproject.org>,
        Dave P Martin <Dave.Martin@arm.com>
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From:   Julien Grall <julien.grall@arm.com>
Message-ID: <cdd5781c-3a60-b9f7-205f-fadeee88206e@arm.com>
Date:   Tue, 26 Feb 2019 10:26:21 +0000
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Hi,

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.

> 
>> The EOI in our case would be clearing the PENDING state. In a proper level
>> interrupt, the state would stay PENDING if there were more to come. This is
>> not the case with the events and you will lose the interrupt.
>>
>> So I don't think they are proper level interrupts. They have more a
>> semantics of edge interrupts with some property of level (i.e for the
>> mask/unmask).
> 
> OK, I guess it depends on how you look at it, to me event channels are
> maybe quirky level interrupts, but are definitely closer to level than
> edge interrupts, specially taking into account the interrupt injection
> that happens on unmask of a pending line, there's no such thing at all
> with edge interrupts.

Cheers,

-- 
Julien Grall