* Alex Bennée (alex.ben...@linaro.org) 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<MasterReq>, > res: Result<()>, > ) -> Result<()> { > if dbg!(self.reply_ack_enabled) { > let hdr = self.new_reply_header::<VhostUserU64>(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 <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? > > - 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?
I think vhost.rs is being correctly strict - but there would be no harm in it flagging that you'd hit an inconsistency if it finds a need_reply without the feature. > - 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? set_mem_table gives a reply when postcopy is enabled (and then qemu replies to the reply!) but otherwise doesn't. (Note there's an issue opened for .rs to support ADD_MEM_REGION since it's a lot better than SET_MEM_TABLE which has a fixed size table that's small). Dave > 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 > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK