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

[Keiichi Watanabe <keiichiw@chromium.org>]

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On Thu, Feb 25, 2021 at 1:20 PM Dylan Reid <dgreid@chromium.org> wrote:

> On Tue, Feb 23, 2021 at 8:20 AM Michael S. Tsirkin <mst@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?
>
>
Yeah, our team is working on vhost-user virtiofs. I think +Woody Chow
<woodychow@chromium.org> will probably be able to look into this issue.


>
> >
> >
> >
> > >  - 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
> >
> >
> > _______________________________________________
> > Rust-vmm mailing list
> > Rust-vmm@lists.opendev.org
> > http://lists.opendev.org/cgi-bin/mailman/listinfo/rust-vmm
>

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