Re: [PATCH 06/18] virtio_ring: avail event index interface

From: Rusty Russell
Date: Tue May 17 2011 - 20:45:31 EST


On Tue, 17 May 2011 09:00:52 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> On Mon, May 16, 2011 at 03:53:19PM +0930, Rusty Russell wrote:
> > On Sun, 15 May 2011 15:47:27 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > > On Mon, May 09, 2011 at 01:43:15PM +0930, Rusty Russell wrote:
> > > > On Wed, 4 May 2011 23:51:19 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > > > > #define VIRTIO_RING_F_USED_EVENT_IDX 29
> > > > > +/* The Host publishes the avail index for which it expects a kick
> > > > > + * at the end of the used ring. Guest should ignore the used->flags field. */
> > > > > +#define VIRTIO_RING_F_AVAIL_EVENT_IDX 32
> > > >
> > > > Are you really sure we want to separate the two? Seems a little simpler
> > > > to have one bit to mean "we're publishing our threshold". For someone
> > > > implementing this from scratch, it's a little simpler.
> > > >
> > > > Or are there cases where the old style makes more sense?
> > > >
> > > > Thanks,
> > > > Rusty.
> > >
> > > Hmm, it makes debugging easier as each side can disable
> > > publishing separately - I used it all the time when I saw
> > > e.g. networking stuck to guess whether I need to investigate the
> > > interrupt or the exit handling.
> > >
> > > But I'm not hung up on this.
> > >
> > > Let me know pls.
> >
> > If we combine them into one, then these patches no longer depend on
> > the feature bit expansion, which is worthwhile (though I'll take both).
> >
> > Thanks,
> > Rusty.
>
> Yes, I know. But if we do expand feature bits anyway, for debugging
> and profiling if nothing else it's useful to have them separate ...
> If you take both why does the order matter?

Damage control. Then if something breaks, it doesn't break everything.

Cheers,
Rusty.
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