On 09/23/2013 03:16 PM, Michael S. Tsirkin wrote: > On Thu, Sep 05, 2013 at 10:54:44AM +0800, Jason Wang wrote: >> > On 09/04/2013 07:59 PM, Michael S. Tsirkin wrote: >>> > > On Mon, Sep 02, 2013 at 04:40:59PM +0800, Jason Wang wrote: >>>> > >> 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@xxxxxxxxxx> >>>> > >> --- >>>> > >> 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. >>> > > >>> > > 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
