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SubjectRe: [PATCH 5 of 5] virtio: expose added descriptors immediately
On Wed, Nov 16, 2011 at 10:51:26AM +1030, Rusty Russell wrote:
> On Mon, 14 Nov 2011 08:56:06 +0200, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > On Sun, Nov 13, 2011 at 11:03:13PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 03, 2011 at 06:12:53PM +1030, Rusty Russell wrote:
> > > > A virtio driver does virtqueue_add_buf() multiple times before finally
> > > > calling virtqueue_kick(); previously we only exposed the added buffers
> > > > in the virtqueue_kick() call. This means we don't need a memory
> > > > barrier in virtqueue_add_buf(), but it reduces concurrency as the
> > > > device (ie. host) can't see the buffers until the kick.
> > > >
> > > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > >
> > > In the past I played with a patch like this, but I didn't see a
> > > performance gain either way. Do you see any gain?
> > >
> > > I'm a bit concerned that with this patch, a buggy driver that
> > > adds more than 2^16 descriptors without a kick
> > > would seem to work sometimes. Let's add WARN_ON(vq->num_added > (1 << 16))?
> >
> > Thinking about it more - it might be tricky for drivers
> > to ensure this. add used to fail when vq is full, but now
> > driver might do get between add and notify:
> > lock
> > add_buf * N
> > prep
> > unlock
> > lock
> > get_buf * N
> > unlock
> > lock
> > add_buf
> > prep
> > unlock
> > notify
> >
> > and since add was followed by get, this doesn't fail.
>
> Right, the driver could, in theory, do:
> add_buf()
> if (!get_buf())
> notify()
>
> But we don't allow that at the moment in our API: we insist on a notify
> occasionally. Noone does this at the moment, so a WARN_ON is correct.
>
> If you're just add_buf() without the get_buf() then add_buf() will fail
> already.
>
> Here's my current variant:
>
> 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
> @@ -245,9 +245,19 @@ add_head:
>
> /* Put entry in available array (but don't update avail->idx until they
> * do sync). */
> - avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
> + avail = (vq->vring.avail->idx & (vq->vring.num-1));
> vq->vring.avail->ring[avail] = head;
>
> + /* Descriptors and available array need to be set before we expose the
> + * new available array entries. */
> + virtio_wmb();
> + 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);
> +
> pr_debug("Added buffer head %i to %p\n", head, vq);
> END_USE(vq);
>
> It's hard to write a useful WARN_ON() for the "you should kick more
> regularly" case (we could take timestamps if DEBUG is defined, I guess),
> so let's leave this until someone actually trips it.
>
> Thanks,
> Rusty.

My unlocked kick patches will trip this warning: they make
virtio-net do add + get without kick.

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;

--
MST


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