From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52621) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1agB8R-0005IT-7h for qemu-devel@nongnu.org; Wed, 16 Mar 2016 09:05:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1agB8M-0001uU-QK for qemu-devel@nongnu.org; Wed, 16 Mar 2016 09:05:11 -0400 Received: from e06smtp12.uk.ibm.com ([195.75.94.108]:37752) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1agB8M-0001qh-IX for qemu-devel@nongnu.org; Wed, 16 Mar 2016 09:05:06 -0400 Received: from localhost by e06smtp12.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 16 Mar 2016 13:05:04 -0000 Date: Wed, 16 Mar 2016 14:04:58 +0100 From: Cornelia Huck Message-ID: <20160316140458.44dcd97d.cornelia.huck@de.ibm.com> In-Reply-To: <56E95646.8020802@redhat.com> References: <1458123018-18651-1-git-send-email-famz@redhat.com> <56E9355A.5070700@redhat.com> <56E93A22.1080102@de.ibm.com> <56E93ECE.10103@redhat.com> <20160316123213.3dcf0abc.cornelia.huck@de.ibm.com> <56E94806.7060505@redhat.com> <20160316125623.38ab4c7e.cornelia.huck@de.ibm.com> <56E94AA9.8000803@redhat.com> <20160316132212.674f766f.cornelia.huck@de.ibm.com> <56E9527B.9010001@redhat.com> <20160316134257.72d7da56.cornelia.huck@de.ibm.com> <56E95646.8020802@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 0/4] Tweaks around virtio-blk start/stop List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Kevin Wolf , Fam Zheng , qemu-block@nongnu.org, "Michael S. Tsirkin" , qemu-devel@nongnu.org, Christian Borntraeger , tubo@linux.vnet.ibm.com, Stefan Hajnoczi On Wed, 16 Mar 2016 13:49:10 +0100 Paolo Bonzini wrote: > On 16/03/2016 13:42, Cornelia Huck wrote: > > On Wed, 16 Mar 2016 13:32:59 +0100 > > Paolo Bonzini wrote: > > > >> On 16/03/2016 13:22, Cornelia Huck wrote: > >>>>> Yeah, it doesn't help that the functions are underdocumented (as in the > >>>>> "assign" parameter above). > >>> My understanding is: > >>> > >>> - 'assign': set up a new notifier (true) or disable it (false) > >>> - 'set_handler': use our handler (true) or have it handled elsewhere > >>> (false) > >> > >> Right. So if we're setting up a new notifier in > >> virtio_queue_aio_set_host_notifier_handler, virtio_pci_stop_ioeventfd should > > ... not call virtio_queue_host_notifier_read. This needs to be cascaded into virtio_queue_set_host_notifier_fd_handler(), I guess. > > >>>>> I don't think the ->set_host_notifiers() api really allows for this. > >>>> > >>>> I think it does, assign is the last argument to k->set_host_notifier(). > >>> > >>> This depends on whether we want 'assign' to clean up any old notifiers > >>> before setting up new ones. I think we want different behaviour for > >>> dataplane and vhost. > >> > >> I think dataplane and vhost are the same. > >> > >> The question is whether ioeventfd=off,vhost=on or > >> ioeventfd=off,dataplane=on are valid combinations; I think they aren't. > > > > We should disallow that even temporary, then. > > > > (This would imply that we need to drop the _stop_ioeventfd() call in > > ->set_host_notifier(), correct?) > > No, it would not. ioeventfd=off,vhost=on would mean: "when vhost is > off, use vCPU thread notification". *confused* Is ioeventfd=off supposed to mean "don't talk to the kernel, do everything in qemu"? > > When turning on vhost you'd still stop ioeventfd (i.e. stop processing > the virtqueue in QEMU's main iothread), but you don't need to do > anything to the event notifier. vhost will pick it up and work on the > virtqueue if necessary. Likewise for dataplane. So "disassociate the handler and switch over to the new one"? > > >> If they aren't, it should be okay to remove the > >> virtio_queue_host_notifier_read call in > >> virtio_queue_set_host_notifier_fd_handler and > >> virtio_queue_aio_set_host_notifier_handler. That's because a handler > >> for the notifier will always be set _somewhere_. It could be the usual > >> ioeventfd handler, the vhost handler or the dataplane handler, but one > >> will be there. > > > > It should; but we probably need to do a final read when we stop the > > ioeventfd. > > I was thinking of handing the final read directly to the next guy who > polls the event notifier instead. So, when called from vhost or > dataplane, virtio_pci_stop_ioeventfd would use > assign=true/set_handler=false ("a new notifier is going to be set up by > the caller"). OK, then we'd need to pass a new parameter for this. > > The host notifier API unfortunately is full of indirections. I'm not > sure how many of them are actually necessary. Oh yes, it's very hard to follow, especially with not-very-well defined parameters.