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

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Message-ID: <87wrhona4n.fsf@rustcorp.com.au>
Date:	Wed, 18 May 2011 09:38:24 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Shirley Ma <xma@...ibm.com>, lguest@...ts.ozlabs.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-s390@...r.kernel.org, kvm@...r.kernel.org,
	Krishna Kumar <krkumar2@...ibm.com>,
	Tom Lendacky <tahm@...ux.vnet.ibm.com>, steved@...ibm.com,
	habanero@...ux.vnet.ibm.com
Subject: Re: [PATCH 06/18] virtio_ring: avail event index interface

On Tue, 17 May 2011 09:00:52 +0300, "Michael S. Tsirkin" <mst@...hat.com> 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@...hat.com> 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@...hat.com> 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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