Re: [Qemu-devel] [Qemu-block] [PATCH 2/3] xen-bus: allow AioContext to be specified for each event channel

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
[snip]
> > > I wonder if the `'is_external' parameter of aio_set_fd_handler shoud be
> > > `true' here, instead. That flag seems to be used when making a snapshot
> > > of a blockdev, for example.
> > >
> > > That was introduced by:
> > > dca21ef23ba48f6f1428c59f295a857e5dc203c8^..c07bc2c1658fffeee08eb46402b2f66d55b07586
> > >
> > > What do you think?
> >
> > Interesting. I admit I was merely transcribing what qemu_set_fd_handler() passes without really
> looking into the values. Looking at the arguments that virtio-blk passes to aio_set_event_notifier()
> though, and what 'is_external' means, it would appear that setting it to true is probably the right
> thing to do. Do you want me to send a v2 of the series or can you fix it up?
> 
> Hi,
> Handlers are invoked by the aio_poll() event loop.  Some handlers are
> considered "external" in the sense that they submit new I/O requests
> from the guest or outside world.  Others are considered "internal" in
> the sense that they are part of the block layer and not an entry point
> into the block layer.
> 
> There are points where the block layer wants to run the event loop but
> new requests must not be submitted.  In this case aio_disable_external()
> will be called so that "external" handlers are not processed.
> 
> For example, see virtio's virtio_queue_aio_set_host_notifier_handler().
> This is the virtqueue kick ioeventfd and it shouldn't be processed when
> aio_disable_external() has been called.
> 

Thanks for the explanation Stefan. Xen event channels/shared rings should indeed be considered as external sources.

  Cheers,

    Paul