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Date: Wed, 3 Jun 2015 15:35:18 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 06/13] tap: Drop tap_can_send
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, Juan Quintela <quintela@redhat.com>, Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org, Vincenzo Maffione <v.maffione@gmail.com>, "Vassili Karpov (malc)" <av1474@comtv.ru>, Gerd Hoffmann <kraxel@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Amit Shah <amit.shah@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Giuseppe Lettieri <g.lettieri@iet.unipi.it>, Luigi Rizzo <rizzo@iet.unipi.it>

On Tue, 06/02 17:21, Stefan Hajnoczi wrote:
> On Tue, May 19, 2015 at 10:51:03AM +0000, Fam Zheng wrote:
> > -    while (qemu_can_send_packet(&s->nc)) {
> > +    while (true) {
> > +        bool can_send;
> >          uint8_t *buf = s->buf;
> >  
> > +        can_send = qemu_can_send_packet(&s->nc);
> > +
> >          size = tap_read_packet(s->fd, s->buf, sizeof(s->buf));
> >          if (size <= 0) {
> >              break;
> > @@ -204,8 +198,12 @@ static void tap_send(void *opaque)
> >              size -= s->host_vnet_hdr_len;
> >          }
> >  
> > +        /* If !can_send, we will want to disable the read poll, but we still
> > +         * need the send completion callback to enable it again, which is a
> > +         * sign of peer becoming ready.  So call the send function anyway.
> > +         */
> >          size = qemu_send_packet_async(&s->nc, buf, size, tap_send_completed);
> > -        if (size == 0) {
> > +        if (size == 0 || !can_send) {
> >              tap_read_poll(s, false);
> >              break;
> >          } else if (size < 0) {
> 
> Why is can_send necessary here but not for the other netdevs that you
> converted?  I thought relying on size == 0 is enough.

I think so. Jason pointed out this offline as well. I'll drop it.

Fam