From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52624) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f6J7G-0005jC-DA for qemu-devel@nongnu.org; Wed, 11 Apr 2018 13:01:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f6J7A-0001Us-CV for qemu-devel@nongnu.org; Wed, 11 Apr 2018 13:01:02 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:34428 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1f6J7A-0001U4-7d for qemu-devel@nongnu.org; Wed, 11 Apr 2018 13:00:56 -0400 Date: Wed, 11 Apr 2018 20:00:53 +0300 From: "Michael S. Tsirkin" Message-ID: <20180411195957-mutt-send-email-mst@kernel.org> References: <20180411072027.5656-1-tiwei.bie@intel.com> <20180411083853.qa6y6zevjpvamrdx@debian> 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] [RFC] vhost-user: introduce F_NEED_ALL_IOTLB protocol feature List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jason Wang Cc: Tiwei Bie , cunming.liang@intel.com, qemu-devel@nongnu.org, peterx@redhat.com, zhihong.wang@intel.com, dan.daly@intel.com On Wed, Apr 11, 2018 at 09:41:05PM +0800, Jason Wang wrote: >=20 >=20 > On 2018=E5=B9=B404=E6=9C=8811=E6=97=A5 16:38, Tiwei Bie wrote: > > On Wed, Apr 11, 2018 at 04:01:19PM +0800, Jason Wang wrote: > > > On 2018=E5=B9=B404=E6=9C=8811=E6=97=A5 15:20, Tiwei Bie wrote: > > > > This patch introduces VHOST_USER_PROTOCOL_F_NEED_ALL_IOTLB > > > > feature for vhost-user. By default, vhost-user backend needs > > > > to query the IOTLBs from QEMU after meeting unknown IOVAs. > > > > With this protocol feature negotiated, QEMU will provide all > > > > the IOTLBs to vhost-user backend without waiting for the > > > > queries from backend. This is helpful when using a hardware > > > > accelerator which is not able to handle unknown IOVAs at the > > > > vhost-user backend. > > > >=20 > > > > Signed-off-by: Tiwei Bie > > > > --- > > > > The idea of this patch is to let QEMU push all the IOTLBs > > > > to vhost-user backend without waiting for the queries from > > > > the backend. Because hardware accelerator at the vhost-user > > > > backend may not be able to handle unknown IOVAs. > > > >=20 > > > > This is just a RFC for now. It seems that, it doesn't work > > > > as expected when guest is using kernel driver (To handle > > > > this case, it seems that some RAM regions' events also need > > > > to be listened). Any comments would be appreciated! Thanks! > > > Interesting, a quick question is why this is needed? Can we just us= e exist > > > IOTLB update message? > > Yeah, we are still using the existing IOTLB update messages > > to send the IOTLB messages to backend. The only difference > > is that, QEMU won't wait for the queries before sending the > > IOTLB update messages. >=20 > Yes, my question is not very clear. I mean why must need a new feature = bit? > It looks to me qemu code can work without this. >=20 > Thanks Generally we avoid adding new messages without a protocol feature bit. While careful analysis might sometimes prove it's not a strict requirement, it's just overall a clean and robust approach. > >=20 > > > It looks to me at least kernel does not need this. > > Something similar in kernel vhost is that, for kernel vhost, > > QEMU needs to push the IOTLBs of some ring addrs to kernel > > vhost backend without waiting for the queries. > >=20 > > Best regards, > > Tiwei Bie > >=20 > > > Thanks