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?
>
> 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.
>> + && vhost_net_tx_select_zcopy(net);
>>
>> /* use msg_control to pass vhost zerocopy ubuf info to skb */
>> if (zcopy_used) {
>> + struct ubuf_info *ubuf;
>> + ubuf = nvq->ubuf_info + nvq->upend_idx;
>> +
>> vq->heads[nvq->upend_idx].id = head;
>> - if (!vhost_net_tx_select_zcopy(net) ||
>> - len < VHOST_GOODCOPY_LEN) {
>> - /* copy don't need to wait for DMA done */
>> - vq->heads[nvq->upend_idx].len =
>> - VHOST_DMA_DONE_LEN;
>> - msg.msg_control = NULL;
>> - msg.msg_controllen = 0;
>> - ubufs = NULL;
>> - } else {
>> - struct ubuf_info *ubuf;
>> - ubuf = nvq->ubuf_info + nvq->upend_idx;
>> -
>> - vq->heads[nvq->upend_idx].len =
>> - VHOST_DMA_IN_PROGRESS;
>> - ubuf->callback = vhost_zerocopy_callback;
>> - ubuf->ctx = nvq->ubufs;
>> - ubuf->desc = nvq->upend_idx;
>> - msg.msg_control = ubuf;
>> - msg.msg_controllen = sizeof(ubuf);
>> - ubufs = nvq->ubufs;
>> - kref_get(&ubufs->kref);
>> - }
>> + vq->heads[nvq->upend_idx].len = VHOST_DMA_IN_PROGRESS;
>> + ubuf->callback = vhost_zerocopy_callback;
>> + ubuf->ctx = nvq->ubufs;
>> + ubuf->desc = nvq->upend_idx;
>> + msg.msg_control = ubuf;
>> + msg.msg_controllen = sizeof(ubuf);
>> + ubufs = nvq->ubufs;
>> + kref_get(&ubufs->kref);
>> nvq->upend_idx = (nvq->upend_idx + 1) % UIO_MAXIOV;
>> - } else
>> + } else {
>> msg.msg_control = NULL;
>> + ubufs = NULL;
>> + }
>> /* TODO: Check specific error and bomb out unless ENOBUFS? */
>> err = sock->ops->sendmsg(NULL, sock, &msg, len);
>> if (unlikely(err < 0)) {
>> if (zcopy_used) {
>> - if (ubufs)
>> - vhost_net_ubuf_put(ubufs);
>> + vhost_net_ubuf_put(ubufs);
>> nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
>> % UIO_MAXIOV;
>> }
>> --
>> 1.7.1
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