From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932588AbeCIVgS (ORCPT ); Fri, 9 Mar 2018 16:36:18 -0500 Received: from mail-vk0-f46.google.com ([209.85.213.46]:45680 "EHLO mail-vk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932157AbeCIVgR (ORCPT ); Fri, 9 Mar 2018 16:36:17 -0500 X-Google-Smtp-Source: AG47ELspnAO+zPbSXx+Mn4UJd6TG7vu5PcVXECU58DmPZLiZoHmYamcYmhMDcJTf0CVSeVCZE9hLXA== Date: Fri, 9 Mar 2018 13:36:12 -0800 From: Christoffer Dall To: Marc Zyngier Cc: Shunyong Yang , ard.biesheuvel@linaro.org, will.deacon@arm.com, eric.auger@redhat.com, david.daney@cavium.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Joey Zheng Subject: Re: [RFC PATCH] KVM: arm/arm64: vgic: change condition for level interrupt resampling Message-ID: <20180309213612.GD1917@lvm> References: <1520492490-7943-1-git-send-email-shunyong.yang@hxt-semitech.com> <9ad47673-068e-f732-d2ca-9c76a8fbdfbc@arm.com> <0a15633d-8944-cb9b-3e6b-b08ee5ec42b9@arm.com> <20180308161900.GC1917@lvm> <86r2oubho3.wl-marc.zyngier@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <86r2oubho3.wl-marc.zyngier@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 08, 2018 at 05:28:44PM +0000, Marc Zyngier wrote: > On Thu, 08 Mar 2018 16:19:00 +0000, > Christoffer Dall wrote: > > > > On Thu, Mar 08, 2018 at 11:54:27AM +0000, Marc Zyngier wrote: > > > On 08/03/18 09:49, Marc Zyngier wrote: [...] > > > The state is now pending, we've really EOI'd the interrupt, and > > > yet lr_signals_eoi_mi() returns false, since the state is not 0. > > > The result is that we won't signal anything on the corresponding > > > irqfd, which people complain about. Meh. > > > > So the core of the problem is that when we've entered the guest with > > PENDING+ACTIVE and when we exit (for some reason) we don't signal the > > resamplefd, right? The solution seems to me that we don't ever do > > PENDING+ACTIVE if you need to resample after each deactivate. What > > would be the point of appending a pending state that you only know to be > > valid after a resample anyway? > > The question is then to identify that a given source needs to be > signalled back to VFIO. Calling into the eventfd code on the hot path > is pretty horrid (I'm not sure if we can really call into this with > interrupts disabled, for example). > This feels like a bad layering violation to me as well. > > > > > > > > Example 2: > > > P+A -> guest EOI -> P -> delayed MI -> guest IAR -> A -> MI fires > > > > We could be more clever and do the following calculation on every exit: > > > > If you enter with P, and exit with either A or 0, then signal. > > > > If you enter with P+A, and you exit with either P, A, or 0, then signal. > > > > Wouldn't that also solve it? (Although I have a feeling you'd miss some > > exits in this case). > > I'd be more confident if we did forbid P+A for such interrupts > altogether, as they really feel like another kind of HW interrupt. How about a slightly bigger hammer: Can we avoid doing P+A for level interrupts completely? I don't think that really makes much sense, and I think we simply everything if we just come back out and resample the line. For an edge, something like a network card, there's a potential performance win to appending a new pending state, but I doubt that this is the case for level interrupts. The timer would be unaffected, because it's a HW interrupt. Thanks, -Christoffer