On 2020/6/7 下午9:57, Michael S. Tsirkin wrote:

On Fri, Jun 05, 2020 at 11:40:17AM +0800, Jason Wang wrote:

quoted

On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:

quoted

On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:

quoted

On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:

quoted

With this patch applied, new and old code perform identically.

Lots of extra optimizations are now possible, e.g.
we can fetch multiple heads with copy_from/to_user now.
We can get rid of maintaining the log array.  Etc etc.

Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
Signed-off-by: Eugenio Pérez<eperezma@redhat.com>
Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com (local)
Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
---
    drivers/vhost/test.c  |  2 +-
    drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
    drivers/vhost/vhost.h |  5 ++++-
    3 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
index 9a3a09005e03..02806d6f84ef 100644
--- a/drivers/vhost/test.c
+++ b/drivers/vhost/test.c

@@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
    	dev = &n->dev;
    	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
    	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
-	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
+	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
    		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);
    	f->private_data = n;

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 8f9a07282625..aca2a5b0d078 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
    {
    	vq->num = 1;
    	vq->ndescs = 0;
+	vq->first_desc = 0;
    	vq->desc = NULL;
    	vq->avail = NULL;
    	vq->used = NULL;

@@ -367,6 +368,11 @@ static int vhost_worker(void *data)
    	return 0;
    }
+static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
+{
+	return vq->max_descs - UIO_MAXIOV;
+}

1 descriptor does not mean 1 iov, e.g userspace may pass several 1 byte
length memory regions for us to translate.

Yes but I don't see the relevance. This tells us how many descriptors to
batch, not how many IOVs.

Yes, but questions are:

- this introduce another obstacle to support more than 1K queue size
- if we support 1K queue size, does it mean we need to cache 1K descriptors,
which seems a large stress on the cache

Thanks

Still don't understand the relevance. We support up to 1K descriptors
per buffer just for IOV since we always did. This adds 64 more
descriptors - is that a big deal?

If I understanding correctly, for net, the code tries to batch 
descriptors for at last one packet.

If we allow 1K queue size then we allow a packet that consists of 1K 
descriptors. Then we need to cache 1K descriptors.

Thanks

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