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

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Message-ID: <20110515124727.GA24932@redhat.com>
Date:	Sun, 15 May 2011 15:47:27 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
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 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 a new feature bit for the host to
> > declare that it uses an avail_event index
> > (like Xen) instead of a feature bit
> > to enable/disable interrupts.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  include/linux/virtio_ring.h |   11 ++++++++---
> >  1 files changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> > index f5c1b75..f791772 100644
> > --- a/include/linux/virtio_ring.h
> > +++ b/include/linux/virtio_ring.h
> > @@ -32,6 +32,9 @@
> >  /* The Guest publishes the used index for which it expects an interrupt
> >   * at the end of the avail ring. Host should ignore the avail->flags field. */
> >  #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.

-- 
MST
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