From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757298AbeDXLzm (ORCPT ); Tue, 24 Apr 2018 07:55:42 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:58356 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753120AbeDXLzh (ORCPT ); Tue, 24 Apr 2018 07:55:37 -0400 Date: Tue, 24 Apr 2018 13:55:33 +0200 From: Cornelia Huck To: Pierre Morel Cc: Dong Jia Shi , pasic@linux.vnet.ibm.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH 01/10] vfio: ccw: Moving state change out of IRQ context Message-ID: <20180424135533.2e9d03dc.cohuck@redhat.com> In-Reply-To: References: <1524149293-12658-1-git-send-email-pmorel@linux.vnet.ibm.com> <1524149293-12658-2-git-send-email-pmorel@linux.vnet.ibm.com> <20180424065442.GV12194@bjsdjshi@linux.vnet.ibm.com> <20180424115929.5b5e1ff0.cohuck@redhat.com> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 24 Apr 2018 13:49:14 +0200 Pierre Morel wrote: > On 24/04/2018 11:59, Cornelia Huck wrote: > > On Tue, 24 Apr 2018 10:40:56 +0200 > > Pierre Morel wrote: > > > >> On 24/04/2018 08:54, Dong Jia Shi wrote: > >>> * Pierre Morel [2018-04-19 16:48:04 +0200]: > >>> > >>> [...] > >>> > >>>> @@ -94,9 +83,15 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work) > >>>> static void vfio_ccw_sch_irq(struct subchannel *sch) > >>>> { > >>>> struct vfio_ccw_private *private = dev_get_drvdata(&sch->dev); > >>>> + struct irb *irb = this_cpu_ptr(&cio_irb); > >>>> > >>>> inc_irq_stat(IRQIO_CIO); > >>>> - vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_INTERRUPT); > >>>> + memcpy(&private->irb, irb, sizeof(*irb)); > >>>> + > >>>> + WARN_ON(work_pending(&private->io_work)); > >>> Hmm, why do we need this? > >> The current design insure that we have not two concurrent SSCH requests. > >> How ever I want here to track spurious interrupt. > >> If we implement cancel, halt or clear requests, we also may trigger (AFAIU) > >> a second interrupts depending on races between instructions, controller > >> and device. > > You won't get an interrupt for a successful cancel. If you do a > > halt/clear, you will make the subchannel halt/clear pending in addition > > to start pending and you'll only get one interrupt (if the I/O has > > progressed far enough, you won't be able to issue a hsch). The > > interesting case is: > > - guest does a ssch, we do a ssch on the device > > - the guest does a csch before it got the interrupt for the ssch > > - before we do the csch on the device, the subchannel is already status > > pending with completion of the ssch > > - after we issue the csch, we get a second interrupt (for the csch) > > We agree. > > > > > I think we should present two interrupts to the guest in that case. > > Races between issuing ssch/hsch/csch and the subchannel becoming status > > pending happen on real hardware as well, we're just more likely to see > > them with the vfio layer in between. > > Yes, agreed too. > > > > > (I'm currently trying to recall what we're doing with unsolicited > > interrupts. These are fun wrt deferred cc 1; I'm not sure if there are > > cases where we want to present a deferred cc to the guest.) > > This patch does not change the current functionalities, only > consolidates the FSM. > The current way to handle unsolicited interrupts is to report them to > the guest > along with the deferred code AFAIU. My question was more along the line of "do we actually want to _generate_ a deferred cc1 or unsolicited interrupt, based upon what we do in our state machine". My guess is no, regardless of the changes you do in this series. > > > > > Also, doing a second ssch before we got final state for the first one > > is perfectly valid. Linux just does not do it, so I'm not sure if we > > should invest too much time there. > > I agree too, it would just make things unnecessary complicated. I'm a big fan of just throwing everything at the hardware and let it sort out any races etc. We just need to be sure we don't mix up interrupts :)