From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:35603) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gkLHC-0002tC-7J for qemu-devel@nongnu.org; Thu, 17 Jan 2019 22:57:03 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gkLHB-0006Tx-3n for qemu-devel@nongnu.org; Thu, 17 Jan 2019 22:57:02 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58890) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gkLHA-0006Iw-RN for qemu-devel@nongnu.org; Thu, 17 Jan 2019 22:57:01 -0500 Date: Thu, 17 Jan 2019 22:56:50 -0500 From: "Michael S. Tsirkin" Message-ID: <20190117225538-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: Yongji Xie Cc: Jason Wang , =?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 Fri, Jan 18, 2019 at 11:32:03AM +0800, Yongji Xie wrote: > On Thu, 17 Jan 2019 at 17:57, Jason Wang wrote: > > > > > > On 2019/1/15 =E4=B8=8B=E5=8D=8810:51, Yongji Xie wrote: > > >> Well, this may work but here're my points: > > >> > > >> 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 > > >> > > >> 2) Since the backend could be killed at any time, the backend must= have > > >> the ability to recover from the partial inflight state > > >> > > >> So it looks to me 1) tends to be self-contradictory and 2) tends t= o be > > >> recursive. The above lines show how tricky could the code looks li= ke. > > >> > > >> Solving this at vhost-user level through at backend is probably wr= ong. > > >> It's time to consider the support from virtio itself. > > >> > > > I agree that supporting this in virtio level may be better. For > > > example, resubmitting inflight I/O once DEVICE_NEEDS_RESET is set i= n > > > Stefan's proposal. But I still think QEMU should be able to provide > > > this ability too. Supposed that one vhost-user backend need to supp= ort > > > multiple VMs. We can't enable reconnect ability until all VMs' gues= t > > > driver support the new feature. It's limited. > > > > > > That's the way virtio evolves. > > > > > > > But if QEMU have the > > > ability to store inflight buffer, the backend could at least have a > > > chance to support this case. > > > > > > The problem is, you need a careful designed protocol described somewh= ere >=20 > That's what we should discuss in detail in this series. >=20 > > (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 > I think the duplicated code is to maintain the inflight descriptor > list which should be in backend. That's not main work in this series. > And backend could choose to include it or not. >=20 > Thanks, > Yongji It would be if someone volunteered to rewrite the vhost user informal description that we have in qemu and make it a full spec. So far a text + implementation in contrib seems plenty to me. --=20 MST