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Date: Tue, 26 May 2015 14:52:48 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v3 03/13] l2tpv3: Drop
	l2tpv3_can_send
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: 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, 05/19 15:48, Stefan Hajnoczi wrote:
> On Tue, May 19, 2015 at 10:51:00AM +0000, Fam Zheng wrote:
> > This callback is called by main loop before polling s->fd, if it returns
> > false, the fd will not be polled in this iteration.
> > 
> > This is redundant with checks inside read callback. After this patch,
> > the data will be copied from s->fd to s->msgvec when it arrives. If the
> > device can't receive, it will be queued to incoming_queue, and when the
> > device status changes, this queue will be flushed.
> 
> This doesn't work because s->msgvec can fill up when
> qemu_can_send_packet() returns false.  At that point we burn 100% CPU
> because the file descriptor is still being monitored.

If qemu_can_send_packet returns false, we do stop monitoring the fd. In
net_l2tpv3_process_queue:

    size = qemu_send_packet_async(
            &s->nc,
            vec->iov_base,
            data_size,
            l2tpv3_send_completed
        );
    if (size == 0) {
        l2tpv3_read_poll(s, false);
    }

The packet is queued and size is 0, so the read poll will be disabled until
it's flushed.

What am I missing?

Fam