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

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Message-ID: <20111122062907.GB11012@redhat.com>
Date:	Tue, 22 Nov 2011 08:29:08 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
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 Tue, Nov 22, 2011 at 11:03:04AM +1030, Rusty Russell wrote:
> 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))

This is 0xffff but why use the decimal notation?

> +		virtqueue_kick(_vq);
>  
>  	pr_debug("Added buffer head %i to %p\n", head, vq);
>  	END_USE(vq);

We also still need to reset vq->num_added, right?

-- 
MST
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