From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:40333) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UahVY-000536-PN for qemu-devel@nongnu.org; Fri, 10 May 2013 03:12:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UahVU-0003Cy-OC for qemu-devel@nongnu.org; Fri, 10 May 2013 03:12:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:23450) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UahVU-0003Ci-HS for qemu-devel@nongnu.org; Fri, 10 May 2013 03:12:44 -0400 Date: Fri, 10 May 2013 09:12:38 +0200 From: Stefan Hajnoczi Message-ID: <20130510071238.GA32616@stefanha-thinkpad.redhat.com> References: <1368060022-16911-1-git-send-email-qemulist@gmail.com> <20130509083017.GH1074@stefanha-thinkpad.redhat.com> <20130509152642.GC26728@stefanha-thinkpad.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH 1/2] Vring: vring's listener's priority should higher than kvm List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: liu ping fan Cc: Peter Maydell , "Michael S. Tsirkin" , Stefan Hajnoczi , qemu-devel@nongnu.org, Anthony Liguori , Jan Kiszka , Paolo Bonzini On Fri, May 10, 2013 at 02:03:34PM +0800, liu ping fan wrote: > On Thu, May 9, 2013 at 11:26 PM, Stefan Hajnoczi wrote: > > On Thu, May 09, 2013 at 05:00:20PM +0800, liu ping fan wrote: > >> On Thu, May 9, 2013 at 4:30 PM, Stefan Hajnoczi wrote: > >> > On Thu, May 09, 2013 at 08:40:21AM +0800, Liu Ping Fan wrote: > >> >> From: Liu Ping Fan > >> >> > >> >> Hosts threads which handle vring should have high MemoryListener priority > >> >> than kvm. For currently code, take the following scenario: > >> >> kvm_region_add() run earlier before vhost_region_add(), then in guest, > >> >> vring's desc[i] can refer to addressX in the new region known by guest. > >> >> But vhost does not know this new region yet, and the vring handler will > >> >> fail. > >> > > >> > Is there a concrete scenario where this happens? > >> > > >> > I can think of situations like the ioeventfd being readable before > >> > vhost/hostmem is populated. But I don't see how that's related to the > >> > priority of kvm_region_add(). > >> > > >> For kvm, ie, In guest, vring_desc.addr can point to a chunk of data in > >> the new added memory, and kick vhost. The vhost has not added this new > >> region, so its local lookup table can not translate this new address, > >> and vring handler will fail. If vhost priority is higher than kvm, > >> then, it will know this new address earlier than kvm. > > > > Isn't the real solution to ensure that the memory API is up-to-date > > before we notify the guest of memory hotplug? > > > No, it is not. > > > I still don't see a kvm vs vhost race. I see a guest vs vhost race > > which priority doesn't fix. > > > Yes, you are right. > The priority should be vhost > guest, and kvm > guest. So vhost == kvm > is OK. But can it be higher or why chosen as 10 not zero? > > If the dependency only lies between MemoryListeners and guest, not > between listeners, then is the priority meanless? I think we should > make sure about this, because if converting core listener to rcu > style, we will definitely break the sequence of region_add/del, ie > both add&del comes after kvm. Okay, so now we're left with the question "what are the ordering dependencies between memory listeners?". I poked around with git-blame(1) but didn't find an explanation. The best I can come up with is that the core listeners in exec.c update QEMU's guest RAM and I/O port mappings, kvm/vhost/xen should be able to query them. Therefore exec.c listeners have priority 0 or 1. BTW the commit that introduced priorities is: commit 72e22d2fe17b85e56b4f0c437c61c6e2de97b308 Author: Avi Kivity Date: Wed Feb 8 15:05:50 2012 +0200 memory: switch memory listeners to a QTAILQ Stefan