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@xxxxxxxxxxxxxxx>
Looking at recent mm compaction patches made me look at locking
in balloon closely. And I noticed the referenced patch (commit
ee7cd8981e15bcb365fc762afe3fc47b8242f630 upstream) interacts strangely
with virtio balloon; balloon currently does:
static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
{
struct scatterlist sg;
sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
init_completion(&vb->acked);
/* We should always be able to add one buffer to an empty queue. */
if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
BUG();
virtqueue_kick(vq);
/* When host has read buffer, this completes via balloon_ack */
wait_for_completion(&vb->acked);
}
While vq callback does:
static void balloon_ack(struct virtqueue *vq)
{
struct virtio_balloon *vb;
unsigned int len;
vb = virtqueue_get_buf(vq, &len);
if (vb)
complete(&vb->acked);
}
So virtqueue_get_buf might now run concurrently with virtqueue_kick.
I audited both and this seems safe in practice but I think
we need to either declare this legal at the API level
or add locking in driver.
Further, is there a guarantee that we never get
spurious callbacks? We currently check ring not empty
but esp for non shared MSI this might not be needed.
If a spurious callback triggers, virtqueue_get_buf can run
concurrently with virtqueue_add_buf which is known to be racy.
Again I think this is currently safe as no spurious callbacks in
practice but should we guarantee no spurious callbacks at the API level
or add locking in driver?
--
MST
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
