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

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Message-ID: <87aaewh5pg.fsf@rustcorp.com.au>
Date:	Mon, 09 May 2011 13:43:15 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:	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,
	linux-kernel@...r.kernel.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 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.
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