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

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Message-ID: <8739dib5z6.fsf@rustcorp.com.au>
Date:	Mon, 21 Nov 2011 12:18:45 +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 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 :)

> 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:

From: Rusty Russell <rusty@...tcorp.com.au>
Subject: virtio: add debugging if driver doesn't kick.

Under the existing #ifdef DEBUG, check that they don't have more than
1/10 of a second between an add_buf() and a
virtqueue_notify()/virtqueue_kick_prepare() call.

We could get false positives on a really busy system, but good for
development.

Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
---
 drivers/virtio/virtio_ring.c |   31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

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
@@ -22,6 +22,7 @@
 #include <linux/device.h>
 #include <linux/slab.h>
 #include <linux/module.h>
+#include <linux/hrtimer.h>
 
 /* virtio guest is communicating with a virtual "device" that actually runs on
  * a host processor.  Memory barriers are used to control SMP effects. */
@@ -102,6 +103,10 @@ struct vring_virtqueue
 #ifdef DEBUG
 	/* They're supposed to lock for us. */
 	unsigned int in_use;
+
+	/* Figure out if their kicks are too delayed. */
+	bool last_add_time_valid;
+	ktime_t last_add_time;
 #endif
 
 	/* Tokens for callbacks. */
@@ -192,6 +197,19 @@ int virtqueue_add_buf(struct virtqueue *
 
 	BUG_ON(data == NULL);
 
+#ifdef DEBUG
+	{
+		ktime_t now = ktime_get();
+
+		/* No kick or get, with .1 second between?  Warn. */
+		if (vq->last_add_time_valid)
+			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
+					    > 100);
+		vq->last_add_time = now;
+		vq->last_add_time_valid = true;
+	}
+#endif
+
 	/* If the host supports indirect descriptor tables, and we have multiple
 	 * buffers, then go indirect. FIXME: tune this threshold */
 	if (vq->indirect && (out + in) > 1 && vq->num_free) {
@@ -291,6 +309,14 @@ bool virtqueue_kick_prepare(struct virtq
 	new = vq->vring.avail->idx;
 	vq->num_added = 0;
 
+#ifdef DEBUG
+	if (vq->last_add_time_valid) {
+		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
+					      vq->last_add_time)) > 100);
+	}
+	vq->last_add_time_valid = false;
+#endif
+
 	if (vq->event) {
 		needs_kick = vring_need_event(vring_avail_event(&vq->vring),
 					      new, old);
@@ -428,6 +454,10 @@ void *virtqueue_get_buf(struct virtqueue
 		virtio_mb();
 	}
 
+#ifdef DEBUG
+	vq->last_add_time_valid = false;
+#endif
+
 	END_USE(vq);
 	return ret;
 }
@@ -611,6 +641,7 @@ struct virtqueue *vring_new_virtqueue(un
 	list_add_tail(&vq->vq.list, &vdev->vqs);
 #ifdef DEBUG
 	vq->in_use = false;
+	vq->last_add_time_valid = false;
 #endif
 
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
--
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