Re: [PATCH RFC 03/13] vhost: batching fetches

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On 2020/6/4 下午4:59, Michael S. Tsirkin wrote:
On Wed, Jun 03, 2020 at 03:27:39PM +0800, Jason Wang wrote:
On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
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@xxxxxxxxxx>
Signed-off-by: Eugenio Pérez<eperezma@xxxxxxxxxx>
Link:https://lore.kernel.org/r/20200401183118.8334-4-eperezma@xxxxxxxxxx
Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>
---
   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







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