On 10/27/2015 04:38 PM, Michael S. Tsirkin wrote:

On Mon, Oct 26, 2015 at 10:52:47AM -0700, Ravi Kerur wrote:

quoted

Ported earlier patch from Jason Wang (dated 12/26/2014).

This patch tries to reduce the number of MSIX irqs required for
virtio-net by sharing a MSIX irq for each TX/RX queue pair through
channels. If transport support channel, about half of the MSIX irqs
were reduced.

Signed-off-by: Ravi Kerur <redacted>

Why bother BTW? 

The reason is we want to save the number of interrupt vectors used.
Booting a guest with 256 queues with current driver will result all
tx/rx queues shares a single vector. This is suboptimal. With this
series, half could be saved. And more complex policy could be applied on
top (e.g limit the number of vectors used by driver).

Looks like this is adding a bunch of overhead
on data path - to what end?

I agree some benchmark is needed for this.

Maybe you have a huge number of these devices ... but in that case, how
about sharing the config interrupt instead?
That's only possible if host supports VIRTIO_1
(so we can detect config interrupt by reading the ISR).

quoted

---
 drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index d8838ded..d705cce 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -72,6 +72,9 @@ struct send_queue {
 
 	/* Name of the send queue: output.$index */
 	char name[40];
+
+	/* Name of the channel, shared with irq. */
+	char channel_name[40];
 };
 
 /* Internal representation of a receive virtqueue */

@@ -1529,6 +1532,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 	int ret = -ENOMEM;
 	int i, total_vqs;
 	const char **names;
+	const char **channel_names;
+	unsigned *channels;
 
 	/* We expect 1 RX virtqueue followed by 1 TX virtqueue, followed by
 	 * possible N-1 RX/TX queue pairs used in multiqueue mode, followed by

@@ -1548,6 +1553,17 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 	if (!names)
 		goto err_names;
 
+	channel_names = kmalloc_array(vi->max_queue_pairs,
+				      sizeof(*channel_names),
+				      GFP_KERNEL);
+	if (!channel_names)
+		goto err_channel_names;
+
+	channels = kmalloc_array(total_vqs, sizeof(*channels),
+				 GFP_KERNEL);
+	if (!channels)
+		goto err_channels;
+
 	/* Parameters for control virtqueue, if any */
 	if (vi->has_cvq) {
 		callbacks[total_vqs - 1] = NULL;

@@ -1562,10 +1578,15 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 		sprintf(vi->sq[i].name, "output.%d", i);
 		names[rxq2vq(i)] = vi->rq[i].name;
 		names[txq2vq(i)] = vi->sq[i].name;
+		sprintf(vi->sq[i].channel_name, "txrx.%d", i);
+		channel_names[i] = vi->sq[i].channel_name;
+		channels[rxq2vq(i)] = i;
+		channels[txq2vq(i)] = i;
 	}
 
 	ret = vi->vdev->config->find_vqs(vi->vdev, total_vqs, vqs, callbacks,
-					 names);
+					 names, channels, channel_names,
+					 vi->max_queue_pairs);
 	if (ret)
 		goto err_find;
 

@@ -1580,6 +1601,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 		vi->sq[i].vq = vqs[txq2vq(i)];
 	}
 
+	kfree(channels);
+	kfree(channel_names);
 	kfree(names);
 	kfree(callbacks);
 	kfree(vqs);

@@ -1587,6 +1610,10 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 	return 0;
 
 err_find:
+	kfree(channels);
+err_channels:
+	kfree(channel_names);
+err_channel_names:
 	kfree(names);
 err_names:
 	kfree(callbacks);

-- 
1.9.1