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

[Dr. David Alan Gilbert]

* Alex Bennée (alex.bennee at 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 / dgilbert at redhat.com / Manchester, UK