> -----Original Message----- > From: Xen-devel [mailto:xen-devel-boun...@lists.xenproject.org] On Behalf Of > Andrew Cooper > Sent: 26 February 2019 09:30 > To: Roger Pau Monne <roger....@citrix.com>; Julien Grall > <julien.gr...@arm.com> > Cc: Juergen Gross <jgr...@suse.com>; Stefano Stabellini > <sstabell...@kernel.org>; Oleksandr > Andrushchenko <andr2...@gmail.com>; linux-kernel@vger.kernel.org; Jan Beulich > <jbeul...@suse.com>; > xen-devel <xen-de...@lists.xenproject.org>; Boris Ostrovsky > <boris.ostrov...@oracle.com>; Dave P > Martin <dave.mar...@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 > > _______________________________________________ > Xen-devel mailing list > xen-de...@lists.xenproject.org > https://lists.xenproject.org/mailman/listinfo/xen-devel
