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

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Date:   Mon, 25 Feb 2019 13:55:42 +0000
From:   Julien Grall <julien.grall@....com>
To:     Oleksandr Andrushchenko <andr2000@...il.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

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?

Cheers,

> [1] 
> https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/io/displif.h;h=cc5de9cb1f35dedc99c866d73d086b19e496852a;hb=HEAD#l756 
> 

-- 
Julien Grall