Re: [PATCH 14/18] virtio: add api for delayed callbacks

From: Michael S. Tsirkin
Date: Thu May 19 2011 - 03:24:46 EST

On Mon, May 16, 2011 at 04:43:21PM +0930, Rusty Russell wrote:
> On Sun, 15 May 2011 15:48:18 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > On Mon, May 09, 2011 at 03:27:33PM +0930, Rusty Russell wrote:
> > > On Wed, 4 May 2011 23:52:33 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > > > Add an API that tells the other side that callbacks
> > > > should be delayed until a lot of work has been done.
> > > > Implement using the new used_event feature.
> > >
> > > Since you're going to add a capacity query anyway, why not add the
> > > threshold argument here?
> >
> > I thought that if we keep the API kind of generic
> > there might be more of a chance that future transports
> > will be able to implement it. For example, with an
> > old host we can't commit to a specific index.
> No, it's always a hint anyway: you can be notified before the threshold
> is reached. But best make it explicit I think.
> Cheers,
> Rusty.

I tried doing that and remembered the real reason I went for this API:

capacity is limited by descriptor table space, not
used ring space: each entry in the used ring frees up multiple entries
in the descriptor ring. Thus the ring can't provide
callback after capacity is N: capacity is only available
after we get bufs.

We could try and make the API pass in the number of freed bufs, however:
- this is not really what virtio-net cares about (it cares about
- if the driver passes a number > number of outstanding bufs, it will
never get a callback. So to stay correct the driver will need to
track number of outstanding requests. The simpler API avoids that.

APIs are easy to change so I'm guessing it's not a major blocker:
we can change later when e.g. block tries to
pass in some kind of extra hint: we'll be smarter
about how this API can change then.


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