On 09/23/2013 03:16 PM, Michael S. Tsirkin wrote:

On Thu, Sep 05, 2013 at 10:54:44AM +0800, Jason Wang wrote:

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On 09/04/2013 07:59 PM, Michael S. Tsirkin wrote:

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On Mon, Sep 02, 2013 at 04:40:59PM +0800, Jason Wang wrote:

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Currently, even if the packet length is smaller than VHOST_GOODCOPY_LEN, if
upend_idx != done_idx we still set zcopy_used to true and rollback this choice
later. This could be avoided by determining zerocopy once by checking all
conditions at one time before.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/vhost/net.c |   47 ++++++++++++++++++++---------------------------
 1 files changed, 20 insertions(+), 27 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 8a6dd0d..3f89dea 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -404,43 +404,36 @@ static void handle_tx(struct vhost_net *net)
 			       iov_length(nvq->hdr, s), hdr_size);
 			break;
 		}
-		zcopy_used = zcopy && (len >= VHOST_GOODCOPY_LEN ||
-				       nvq->upend_idx != nvq->done_idx);
+
+		zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN
+				   && (nvq->upend_idx + 1) % UIO_MAXIOV !=
+				      nvq->done_idx

Thinking about this, this looks strange.
The original idea was that once we start doing zcopy, we keep
using the heads ring even for short packets until no zcopy is outstanding.

What's the reason for keep using the heads ring?

To keep completions in order.

Ok, I will do some test to see the impact.

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What's the logic behind (nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx
here?

Because we initialize both upend_idx and done_idx to zero, so upend_idx
!= done_idx could not be used to check whether or not the heads ring
were full.

But what does ring full have to do with zerocopy use?

It was used to forbid the zerocopy when heads ring are full, but since
we have the limitation now, it could be removed.