Re: [PATCH RFC v2 04/16] vfio-user: connect vfio proxy to remote server

[Stefan Hajnoczi]

On Fri, Sep 10, 2021 at 05:25:13AM +0000, John Johnson wrote:
> 
> 
> > On Sep 8, 2021, at 11:29 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Thu, Sep 09, 2021 at 05:11:49AM +0000, John Johnson wrote:
> >> 
> >> 
> >>    I did look at coroutines, but they seemed to work when the sender
> >> is triggering the coroutine on send, not when request packets are arriving
> >> asynchronously to the sends.
> > 
> > This can be done with a receiver coroutine. Its job is to be the only
> > thing that reads vfio-user messages from the socket. A receiver
> > coroutine reads messages from the socket and wakes up the waiting
> > coroutine that yielded from vfio_user_send_recv() or
> > vfio_user_pci_process_req().
> > 
> > (Although vfio_user_pci_process_req() could be called directly from the
> > receiver coroutine, it seems safer to have a separate coroutine that
> > processes requests so that the receiver isn't blocked in case
> > vfio_user_pci_process_req() yields while processing a request.)
> > 
> > Going back to what you mentioned above, the receiver coroutine does
> > something like this:
> > 
> >  if it's a reply
> >      reply = find_reply(...)
> >      qemu_coroutine_enter(reply->co) // instead of signalling reply->cv
> >  else
> >      QSIMPLEQ_INSERT_TAIL(&pending_reqs, request, next);
> >      if (pending_reqs_was_empty) {
> >          qemu_coroutine_enter(process_request_co);
> >      }
> > 
> > The pending_reqs queue holds incoming requests that the
> > process_request_co coroutine processes.
> > 
> 
> 
>       How do coroutines work across threads?  There can be multiple vCPU
> threads waiting for replies, and I think the receiver coroutine will be
> running in the main loop thread.  Where would a vCPU block waiting for
> a reply?  I think coroutine_yield() returns to its coroutine_enter() caller.

A vCPU thread holding the BQL can iterate the event loop if it has
reached a synchronous point that needs to wait for a reply before
returning. I think we have this situation when a MemoryRegion is
accessed on the proxy device.

For example, block/block-backend.c:blk_prw() kicks off a coroutine and
then runs the event loop until the coroutine finishes:

  Coroutine *co = qemu_coroutine_create(co_entry, &rwco);
  bdrv_coroutine_enter(blk_bs(blk), co);
  BDRV_POLL_WHILE(blk_bs(blk), rwco.ret == NOT_DONE);

BDRV_POLL_WHILE() boils down to a loop like this:

  while ((cond)) {
    aio_poll(ctx, true);
  }

I also want to check that I understand the scenarios in which the
vfio-user communication code is used:

1. vhost-user-server

The vfio-user communication code should run in a given AioContext (it
will be the main loop by default but maybe the user will be able to
configure a specific IOThread in the future).

2. vCPU thread vfio-user clients

The vfio-user communication code is called from the vCPU thread where
the proxy device executes. The MemoryRegion->read()/write() callbacks
are synchronous, so the thread needs to wait for a vfio-user reply
before it can return.

Is this what you had in mind?

Stefan

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