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Date: Wed, 1 Dec 2010 17:42:56 +0530
From: Amit Shah <amit.shah@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v8 7/7] virtio-console: Enable port
	throttling when chardev is slow to consume data
Message-ID: <20101201121256.GD2962@amit-x200.redhat.com>
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	<201012011130.58630.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: Juan Quintela <quintela@redhat.com>, qemu-devel@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>

On (Wed) Dec 01 2010 [11:59:35], Paul Brook wrote:
> > > > -    qemu_chr_write(vcon->chr, buf, len);
> > > > +    ret = qemu_chr_write(vcon->chr, buf, len);
> > > > +    if (ret == -EAGAIN) {
> > > > +        virtio_serial_throttle_port(port, true);
> > > > +    }
> > > > 
> > > >  }
> > > 
> > > This looks wrong. It will loose data in the case of a partial write
> > > (i.e. ret < len)
> > 
> > That doesn't happen currently (qemu_chr_write doesn't return a value > 0
> > but < len).
> > 
> > I had code in there to handle it, but that would change behaviour for
> > current users of qemu_chr_write(), which is a risk.
> 
> Doesn't that make the code almost completely pointless?

Not really -- I did have code for partial writes, but removed it before
this submission (had it in previous versions).

The (new) do_send loop:

    len = len1;
    while (len > 0) {
        ret = write(fd, buf, len);
        if (ret < 0) {
            if (errno == EAGAIN && nonblock) {
                return -EAGAIN;
            }
            if (errno != EINTR && errno != EAGAIN) {
                return -1;
            }
        } else if (ret == 0) {
            break;
        } else {
            buf += ret;
            len -= ret;
        }
    }

when there's a partial write, it tries to do a write again, which will
fail with -EAGAIN.

(However I might be completely missing the data that didn't get written
as a result of that partial write, so the output gets corrupted.  I
guess I still need to handle partial writes for that.)

		Amit