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[79.180.2.31]) by smtp.gmail.com with ESMTPSA id c133sm2478925wme.46.2021.02.23.03.44.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Feb 2021 03:44:58 -0800 (PST) Date: Tue, 23 Feb 2021 06:44:55 -0500 From: "Michael S. Tsirkin" To: Alex =?iso-8859-1?Q?Benn=E9e?= Subject: Re: vhost reply_ack negotiation (a.k.a differences in vhost-user behaviour with libvhost-user and vhost-user-backend.rs) Message-ID: <20210223064312-mutt-send-email-mst@kernel.org> References: <8735xskm7j.fsf@linaro.org> MIME-Version: 1.0 In-Reply-To: <8735xskm7j.fsf@linaro.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Sergio Lopez , "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Stefan Hajnoczi , "rust-vmm@lists.opendev.org" , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , raphael.norwitz@nutanix.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Cc: Raphael On Fri, Feb 19, 2021 at 04:04:34PM +0000, Alex Bennée wrote: > Hi, > > I finally got a chance to get down into the guts of vhost-user while > attempting to port my original C RPMB daemon to Rust using the > vhost-user-backend and related crates. I ended up with this hang during > negotiation: > > startup > > vhost_user_write req:1 flags:0x1 > vhost_user_read_start > vhost_user_read req:1 flags:0x5 > vhost_user_backend_init: we got 170000000 > vhost_user_write req:15 flags:0x1 > vhost_user_read_start > vhost_user_read req:15 flags:0x5 > vhost_user_set_protocol_features: 2008 > vhost_user_write req:16 flags:0x1 > vhost_user_write req:3 flags:0x1 > vhost_user_write req:1 flags:0x1 > vhost_user_read_start > vhost_user_read req:1 flags:0x5 > vhost_user_write req:13 flags:0x1 > > kernel initialises device > > virtio_rpmb virtio1: init done! > vhost_user_write req:13 flags:0x1 > vhost_dev_set_features: 130000000 > vhost_user_set_features: 130000000 > vhost_user_write req:2 flags:0x1 > vhost_user_write req:5 flags:0x9 > vhost_user_read_start > > The proximate cause is the vhost crate handling: > > MasterReq::SET_MEM_TABLE => { > let res = self.set_mem_table(&hdr, size, &buf, rfds); > self.send_ack_message(&hdr, res)?; > } > > which gates on the replay_ack_enabled flag: > > fn send_ack_message( > &mut self, > req: &VhostUserMsgHeader, > res: Result<()>, > ) -> Result<()> { > if dbg!(self.reply_ack_enabled) { > let hdr = self.new_reply_header::(req, 0)?; > let val = match res { > Ok(_) => 0, > Err(_) => 1, > }; > let msg = VhostUserU64::new(val); > self.main_sock.send_message(&hdr, &msg, None)?; > } > Ok(()) > } > > which is only set when we have all the appropriate acknowledged flags: > > fn update_reply_ack_flag(&mut self) { > let vflag = VhostUserVirtioFeatures::PROTOCOL_FEATURES.bits(); > let pflag = VhostUserProtocolFeatures::REPLY_ACK; > if (self.virtio_features & vflag) != 0 > && (self.acked_virtio_features & vflag) != 0 > && self.protocol_features.contains(pflag) > && (self.acked_protocol_features & pflag.bits()) != 0 > { > self.reply_ack_enabled = true; > } else { > self.reply_ack_enabled = false; > } > } > > which from above you can see QEMU helpfully dropped those bits in the > reply. It does however work in the C/libvhost version: > > virtio_rpmb virtio1: init done! > vhost_user_write req:13 flags:0x1 > vhost_dev_set_features: 130000000 > vhost_user_set_features: 130000000 > vhost_user_write req:2 flags:0x1 > vhost_user_write req:37 flags:0x9 > vhost_user_read_start > vhost_user_read req:37 flags:0x5 > vhost_user_write req:8 flags:0x1 > vhost_user_write req:10 flags:0x1 > vhost_user_write req:9 flags:0x1 > vhost_user_write req:12 flags:0x1 > vhost_user_write req:13 flags:0x1 > > albeit with a slightly different message sequence > (VHOST_USER_ADD_MEM_REG instead of VHOST_USER_SET_MEM_TABLE). Reading > the C code you can see why: > > need_reply = vmsg.flags & VHOST_USER_NEED_REPLY_MASK; > > reply_requested = vu_process_message(dev, &vmsg); > if (!reply_requested && need_reply) { > vmsg_set_reply_u64(&vmsg, 0); > reply_requested = 1; > } > > So regardless of what may have been negotiated it will always reply with > something if the master requested it do so. This points us at the > specification which reads: > > - Bit 3 is the need_reply flag - see :ref:`REPLY_ACK ` for > details. > > which says in VHOST_USER_PROTOCOL_F_REPLY_ACK that this bit should only > be honoured when the feature has been negotiated. Which brings us to a > series of questions: > > - Should QEMU have preserved VhostUserVirtioFeatures::PROTOCOL_FEATURES > when doing the eventual VHOST_USER_SET_FEATURES reply? Hmm looks like a bug indeed ... Anyone wants to look into fixing that? Marc-André? > - Is vhost.rs being to strict or libvhost-user too lax in interpreting > the negotiated features before processing the ``need_reply`` [Bit 3] > field of the messages? > > - are VHOST_USER_SET_MEM_TABLE to VHOST_USER_SET_INFLIGHT_FD included > in the "list of the ones that do" require replies or do they only > reply when REPLY_ACK has been negotiated as the ambiguous "seealso::" > box out seems to imply? > > Currently I have some hacks in: > > https://github.com/stsquad/vhost/tree/my-hacks > > which gets my daemon booting up to the point we actually need to do a > transaction. However I won't submit a PR until I've worked out exactly > where the problems are. > > -- > Alex Bennée