From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42580) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Yn3B7-0004cN-1T for qemu-devel@nongnu.org; Tue, 28 Apr 2015 06:55:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Yn3B3-0000FM-Lp for qemu-devel@nongnu.org; Tue, 28 Apr 2015 06:55:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34952) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Yn3B2-0000FB-Sf for qemu-devel@nongnu.org; Tue, 28 Apr 2015 06:55:45 -0400 Date: Tue, 28 Apr 2015 12:55:40 +0200 From: "Michael S. Tsirkin" Message-ID: <20150428124510-mutt-send-email-mst@redhat.com> References: <1429770109-23873-1-git-send-email-jasowang@redhat.com> <1429770109-23873-9-git-send-email-jasowang@redhat.com> <20150427130441-mutt-send-email-mst@redhat.com> <1430190844.9163.3@smtp.corp.redhat.com> <20150428071225-mutt-send-email-mst@redhat.com> <1430201600.5354.0@smtp.corp.redhat.com> <20150428085941-mutt-send-email-mst@redhat.com> <20150428100415.377222a3.cornelia.huck@de.ibm.com> <20150428100706-mutt-send-email-mst@redhat.com> <20150428124007.443a6555.cornelia.huck@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150428124007.443a6555.cornelia.huck@de.ibm.com> Subject: Re: [Qemu-devel] [PATCH V7 08/16] virtio: introduce bus specific queue limit List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck Cc: Jason Wang , qemu-devel@nongnu.org, Alexander Graf , Christian Borntraeger , Paolo Bonzini , Richard Henderson On Tue, Apr 28, 2015 at 12:40:07PM +0200, Cornelia Huck wrote: > On Tue, 28 Apr 2015 10:16:04 +0200 > "Michael S. Tsirkin" wrote: > > > On Tue, Apr 28, 2015 at 10:04:15AM +0200, Cornelia Huck wrote: > > > On Tue, 28 Apr 2015 09:14:07 +0200 > > > "Michael S. Tsirkin" wrote: > > > > > > > On Tue, Apr 28, 2015 at 02:13:20PM +0800, Jason Wang wrote: > > > > > > > > > > > > > > > On Tue, Apr 28, 2015 at 1:13 PM, Michael S. Tsirkin wrote: > > > > > >On Tue, Apr 28, 2015 at 11:14:04AM +0800, Jason Wang wrote: > > > > > >> On Mon, Apr 27, 2015 at 7:05 PM, Michael S. Tsirkin > > > > > >> wrote: > > > > > >> >On Thu, Apr 23, 2015 at 02:21:41PM +0800, Jason Wang wrote: > > > > > >> >> This patch introduces a bus specific queue limitation. It will be > > > > > >> >> useful for increasing the limit for one of the bus without > > > > > >>disturbing > > > > > >> >> other buses. > > > > > >> >> Cc: Michael S. Tsirkin > > > > > >> >> Cc: Alexander Graf > > > > > >> >> Cc: Richard Henderson > > > > > >> >> Cc: Cornelia Huck > > > > > >> >> Cc: Christian Borntraeger > > > > > >> >> Cc: Paolo Bonzini > > > > > >> >> Signed-off-by: Jason Wang > > > > > >> >> Reviewed-by: Cornelia Huck > > > > > >> > > > > > > >> >Is this still needed if you drop the attempt to > > > > > >> >keep the limit around for old machine types? > > > > > >> If we agree to drop, we probably need transport specific macro. > > > > > > > > > > > >You mean just rename VIRTIO_PCI_QUEUE_MAX to VIRTIO_QUEUE_MAX? > > > > > >Fine, why not. > > > > > > > > > > I mean keeping VIRTIO_PCI_QUEUE_MAX for pci only and just increase pci > > > > > limit. And introduce e.g VIRTIO_PCI_QUEUE_CCW for ccw and keep it as 64. > > > > > Since to my understanding, it's not safe to increase the limit for all other > > > > > transports which was pointed out by Cornelia in V1: > > > > > http://permalink.gmane.org/gmane.comp.emulators.qemu/318245. > > > > > > > > I think all you need is add a check to CCW_CMD_SET_IND: > > > > limit to 64 for legacy interrupts only. > > > > > > It isn't that easy. > > > > > > What is easy is to add a check to the guest driver that fails setup for > > > devices with more than 64 queues not using adapter interrupts. > > > > > > On the host side, we're lacking information when interpreting > > > CCW_CMD_SET_IND (the command does not contain a queue count, and the > > > actual number of virtqueues is not readily available.) > > > > Why isn't it available? All devices call virtio_add_queue > > as appropriate. Just fail legacy adaptors. > > Because we don't know what the guest is going to use? It is free to > use per-subchannel indicators, even if it is operating in virtio-1 mode. > > > > > We also can't > > > fence off when setting up the vqs, as this happens before we know which > > > kind of indicators the guest wants to use. > > > > > > More importantly, we haven't even speced what we want to do in this > > > case. Do we want to reject SET_IND for devices with more than 64 > > > queues? (Probably yes.) > > > > > > All this involves more work, and I'd prefer to do Jason's changes > > > instead as this gives us some more time to figure this out properly. > > > > > > And we haven't even considered s390-virtio yet, which I really want to > > > touch as little as possible :) > > > > Well this patch does touch it anyway :) > > But only small, self-evident changes. > Sorry, I don't see what you are trying to say. There's no chance legacy interrupts work with > 64 queues. Guests should have validated the # of queues, and not attempted to use >64 queues. Looks like there's no such validation in guest, right? Solution - don't specify this configuration with legacy guests. Modern guests work so there's value in supporting such configuration in QEMU, I don't see why we must deny it in QEMU. > > For s390 just check and fail at init if you like. > > What about devices that may change their number of queues? I'd really > prefer large queue numbers to be fenced off in the the individual > devices, and for that they need to be able to grab a transport-specific > queue limit. This is why I don't want bus specific limits in core, it just makes it too easy to sweep dirt under the carpet. s390 is legacy - fine, but don't perpetuate the issue in devices. -- MST