[Rust-VMM] vhost reply_ack negotiation (a.k.a differences in vhost-user behaviour with libvhost-user and vhost-user-backend.rs)

[Dylan Reid]

On Tue, Feb 23, 2021 at 8:20 AM Michael S. Tsirkin <mst at redhat.com> wrote:
>
> 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<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?
>
> Hmm looks like a bug indeed ... Anyone wants to look
> into fixing that? Marc-André?

chirantan and keiichi will be implementing vhost-user-vitio-fs on
Chrome OS, maybe one of you two can take a look?


>
>
>
> >  - 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
>
>
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