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

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Date:   Tue, 26 Feb 2019 10:44:59 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
CC:     Julien Grall <julien.grall@....com>,
        Oleksandr Andrushchenko <andr2000@...il.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jan Beulich <JBeulich@...e.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Dave P Martin <Dave.Martin@....com>
Subject: Re: [Xen-devel] xen/evtchn and forced threaded irq

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.

Thanks, Roger.