Re: [Qemu-devel] [RFC] docs: vhost-user: add in-band kick/call messages

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Sep 10, 2019 at 05:14:36PM +0200, Johannes Berg wrote:
> On Tue, 2019-09-10 at 17:03 +0200, Stefan Hajnoczi wrote:
> > 
> > > Now, this means that the CPU (that's part of the simulation) has to
> > > *wait* for the device to add an entry to the simulation calendar in
> > > response to the kick... That means that it really has to look like
> > > 
> > > CPU               device                   calendar
> > >      ---[kick]-->
> > >                          ---[add entry]-->
> > >                          <---[return]-----
> > 
> > What are the semantics of returning from the calendar?  Does it mean
> > "it's now your turn to run?", "your entry has been added and you'll be
> > notified later when it's time to run?", or something else?
> 
> The latter - the entry was added, and you'll be notified when it's time
> to run; but we need to have that state on the calendar so the CPU won't
> converse with the calendar before that state is committed.

Is the device only adding a calendar entry and not doing any actual
device emulation at this stage?

If yes, then this suggests the system could be structured more cleanly.
The vhost-user device process should focus on device emulation.  It
should not be aware of the calendar.  The vhost-user protocol also
shouldn't require modifications.

Instead, Linux arch/um code would add the entry to the calendar when the
CPU wants to kick a vhost-user device.  I assume the CPU is suspended
until arch/um code completes adding the entry to the calendar.

When the calendar decides to run the device entry it signals the
vhost-user kick eventfd.  The vhost-user device processes the virtqueue
as if it had been directly signalled by the CPU, totally unaware that
it's running within a simulation system.

The irq path is similar: the device signals the callfd and the calendar
adds an entry to notify UML that the request has completed.

Some pseudo-code:

arch/um/drivers/.../vhost-user.c:

  void um_vu_kick(struct um_vu_vq *vq)
  {
      if (simulation_mode) {
          calendar_add_entry({
              .type = CAL_ENTRY_TYPE_VHOST_USER_KICK,
              .device = vq->dev,
              .vq_idx = vq->idx,
          });
          return;
      }

      /* The normal code path: signal the kickfd */
      uint64_t val = 1;
      write(vq->kickfd, &val, sizeof(val));
  }

I'm suggesting this because it seems like a cleaner approach than
exposing the calendar concept to the vhost-user devices.  I'm not 100%
sure it offers the semantics you need to make everything deterministic
though.

A different topic: vhost-user does not have a 1:1 vq buffer:kick
relationship.  It's possible to add multiple buffers and kick only once.
It is also possible for the device to complete multiple buffers and only
call once.

This could pose a problem for simulation because it allows a degree of
non-determinism.  But as long as the both the CPU and the I/O completion
of the device happen on a strict schedule this isn't a problem.

Stefan

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