Re: [Qemu-devel] [PATCH 0/9] introduce virtio net dataplane

[Stefan Hajnoczi]

On Thu, Feb 21, 2013 at 05:38:04PM -0600, mdroth wrote:
> On Thu, Feb 21, 2013 at 11:12:01PM +0100, Paolo Bonzini wrote:
> > Il 21/02/2013 22:07, mdroth ha scritto:
> > >> > 
> > >> > 100% agree.  In particular hw/dataplane/event-poll.c should be the 
> > >> > first
> > >> > to go away, but AioContext provides the functionality that Ping Fan
> > >> > needs.  But hw/dataplane/vring.c will probably be here for a longer
> > > Has there been any discussion around introducing something similar to
> > > AioContexts for fd handlers? This would avoid the dataplane-specific hooks
> > > needed for NetClients in this series.
> > 
> > AioContext can include file descriptors on POSIX systems (used for NBD
> > and other network backends), see aio_set_fd_handler.
> 
> Sorry, was using "fd handlers" too generally. I mean specifically for
> the qemu_set_fd_handler interfaces, where we currently rely on a single list
> of IOHandlerRecords for registration and a single loop to drive them. Would
> be nice if we could drive subsets of those via mini main loops, similar to the
> way dataplane threads would with a particular AioContext via aio_poll (or 
> perhaps
> the exact same way)
> 
> Currently, Ping Fan's patches basically do this already by accessing a
> global reference to the vnet worker thread and attaching events/handlers to
> it's event loop via a new set of registration functions (PATCH 7).
> 
> I think generalizing this by basing qemu_set_fd_handler() around
> AioContext, or something similar, would help to extend support to other
> NetClient implementations without requiring dataplane-specific hooks
> throughout.

We used to have a nesting feature that we got rid of.  I don't remember
the details but basically a nesting level counter:

  aio_set_fd_handler(...); /* fd at level N */

  aio_nesting_inc();
  aio_set_fd_handler(...); /* fd at level N+1 */
  aio_poll(); /* just this fd */
  aio_nesting_dec();

  aio_poll(); /* both fds */

Then aio_poll() only considers fds which are greater than or equal to
the current nesting level.

I remember disliking this feature but maybe it was just being used badly
rather than the mechanism itself being bad.

Stefan