RE: [Xen-devel] xen/evtchn and forced threaded irq

From: Paul Durrant <Paul.Durrant@citrix.com>
To: Andrew Cooper <Andrew.Cooper3@citrix.com>,
	Roger Pau Monne <roger.pau@citrix.com>,
	Julien Grall <julien.grall@arm.com>
Cc: Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Oleksandr Andrushchenko <andr2000@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jan Beulich <JBeulich@suse.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	Dave P Martin <Dave.Martin@arm.com>
Subject: RE: [Xen-devel] xen/evtchn and forced threaded irq
Date: Tue, 26 Feb 2019 09:45:50 +0000	[thread overview]
Message-ID: <73014d8f3d234e0ab315f5205d61c8d2@AMSPEX02CL02.citrite.net> (raw)
In-Reply-To: <038b837c-63c0-afb7-ca7b-75f61af7518e@citrix.com>

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@lists.xenproject.org] On Behalf Of Andrew Cooper
> Sent: 26 February 2019 09:30
> To: Roger Pau Monne <roger.pau@citrix.com>; Julien Grall <julien.grall@arm.com>
> Cc: Juergen Gross <jgross@suse.com>; Stefano Stabellini <sstabellini@kernel.org>; Oleksandr
> Andrushchenko <andr2000@gmail.com>; linux-kernel@vger.kernel.org; Jan Beulich <JBeulich@suse.com>;
> xen-devel <xen-devel@lists.xenproject.org>; Boris Ostrovsky <boris.ostrovsky@oracle.com>; Dave P
> Martin <Dave.Martin@arm.com>
> Subject: Re: [Xen-devel] xen/evtchn and forced threaded irq
> 
> 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.

They are more subtle than that are they not? There is a single per-vcpu ACK which can cover multiple event channels.

  Paul

> 
> 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.
> 
> ~Andrew
> 
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