linux-kernel - Re: [Xen-devel] xen/evtchn and forced threaded irq

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Date:   Mon, 25 Feb 2019 16:08:11 +0200
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Julien Grall <julien.grall@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jan Beulich <JBeulich@...e.com>,
        Dave P Martin <Dave.Martin@....com>,
        xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [Xen-devel] xen/evtchn and forced threaded irq

On 2/25/19 3:55 PM, 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, they can be merged so you can get a signal notification to 
> the guest for multiple "events". So if you rely on the event to have 
> an associated payload, then you probably have done something wrong in 
> your driver.
>
> I haven't implemented PV drivers myself, but I would expect either 
> side to block if there were no space in the ring.
>
> What do you do in the displif driver when the ring is full?
>
It is handled by the originator, the display backend in our case: it 
doesn't send
events if it sees that the ring will overflow. But I was worried about
such a generic change with counting number of events received and if this
really helps to recover in general case
> Cheers,
>
>> [1] 
>> https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/io/displif.h;h=cc5de9cb1f35dedc99c866d73d086b19e496852a;hb=HEAD#l756 
>>
>