linux-kernel - Re: [PATCH 5 of 5] virtio: expose added descriptors immediately

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Message-ID: <87lir9xagv.fsf@rustcorp.com.au>
Date:	Tue, 22 Nov 2011 11:03:04 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 5 of 5] virtio: expose added descriptors immediately

On Mon, 21 Nov 2011 13:57:04 +0200, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Mon, Nov 21, 2011 at 12:18:45PM +1030, Rusty Russell wrote:
> > On Wed, 16 Nov 2011 09:18:38 +0200, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > > My unlocked kick patches will trip this warning: they make
> > > virtio-net do add + get without kick.
> > 
> > Heh, it's a good sign if they do, since that means you're running really
> > well :)
> 
> They don't in fact, in my testing :(. But I think they can with luck.
> 
> > > I think block with unlocked kick can trip it too:
> > > add, lock is dropped and then an interrupt can get.
> > > 
> > > We also don't need a kick each num - each 2^15 is enough.
> > > Why don't we do this at start of add_buf:
> > > if (vq->num_added >= 0x7fff)
> > > 	return -ENOSPC;
> > 
> > The warning was there in case a driver is never doing a kick, and
> > getting away with it (mostly) because the device is polling.  Let's not
> > penalize good drivers to catch bad ones.
> > 
> > How about we do this properly, like so:
> 
> Absolutely. But I think we also need to handle num_added
> overflow of a 15 bit counter, no? Otherwise the
> vring_need_event logic might give us false negatives ....
> I'm guessing we can just assume we need a kick in that case.

You're right.  Thankyou.  My immediate reaction of "make it an unsigned
long" doesn't work.

Here's the diff to what I posted before:

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -254,9 +254,10 @@ add_head:
 	vq->vring.avail->idx++;
 	vq->num_added++;
 
-	/* If you haven't kicked in this long, you're probably doing something
-	 * wrong. */
-	WARN_ON(vq->num_added > vq->vring.num);
+	/* This is very unlikely, but theoretically possible.  Kick
+	 * just in case. */
+	if (unlikely(vq->num_added == 65535))
+		virtqueue_kick(_vq);
 
 	pr_debug("Added buffer head %i to %p\n", head, vq);
 	END_USE(vq);
--
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