From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:57761) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gk996-0007hA-QS for qemu-devel@nongnu.org; Thu, 17 Jan 2019 09:59:53 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gk990-0000kh-Vt for qemu-devel@nongnu.org; Thu, 17 Jan 2019 09:59:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38942) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gk990-0000if-K6 for qemu-devel@nongnu.org; Thu, 17 Jan 2019 09:59:46 -0500 Date: Thu, 17 Jan 2019 09:59:39 -0500 From: "Michael S. Tsirkin" Message-ID: <20190117095848-mutt-send-email-mst@kernel.org> References: <20190109112728.9214-1-xieyongji@baidu.com> <20190109112728.9214-5-xieyongji@baidu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH v4 for-4.0 4/7] libvhost-user: Support tracking inflight I/O in shared memory List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jason Wang Cc: Yongji Xie , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , "Coquelin, Maxime" , Yury Kotov , =?utf-8?B?0JXQstCz0LXQvdC40Lkg0K/QutC+0LLQu9C10LI=?= , qemu-devel , zhangyu31@baidu.com, chaiwen@baidu.com, nixun@baidu.com, lilin24@baidu.com, Xie Yongji , Stefan Hajnoczi On Thu, Jan 17, 2019 at 05:57:29PM +0800, Jason Wang wrote: >=20 > On 2019/1/15 =E4=B8=8B=E5=8D=8810:51, Yongji Xie wrote: > > > Well, this may work but here're my points: > > >=20 > > > 1) The code want to recover from backed crash by introducing extra = space > > > to store inflight data, but it still depends on the backend to set/= get > > > the inflight state > > >=20 > > > 2) Since the backend could be killed at any time, the backend must = have > > > the ability to recover from the partial inflight state > > >=20 > > > So it looks to me 1) tends to be self-contradictory and 2) tends to= be > > > recursive. The above lines show how tricky could the code looks lik= e. > > >=20 > > > Solving this at vhost-user level through at backend is probably wro= ng. > > > It's time to consider the support from virtio itself. > > >=20 > > I agree that supporting this in virtio level may be better. For > > example, resubmitting inflight I/O once DEVICE_NEEDS_RESET is set in > > Stefan's proposal. But I still think QEMU should be able to provide > > this ability too. Supposed that one vhost-user backend need to suppor= t > > multiple VMs. We can't enable reconnect ability until all VMs' guest > > driver support the new feature. It's limited. >=20 >=20 > That's the way virtio evolves. >=20 >=20 > > But if QEMU have the > > ability to store inflight buffer, the backend could at least have a > > chance to support this case. >=20 >=20 > The problem is, you need a careful designed protocol described somewher= e (is > vhost-user.txt a good place for this?). And this work will be (partial) > duplicated for the future support from virtio spec itself. >=20 >=20 > > Maybe backend could have other way to > > avoid the tricky code. >=20 >=20 > I'm not sure, but it was probably not easy. I see an implementation in libvhost-user. Do you see an issue there? > Thanks >=20 >=20 > >=20 > > Thanks,