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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH v8 7/7] virtio-console: Enable port
	throttling when chardev is slow to consume data
Date: Wed, 1 Dec 2010 13:08:26 +0000
References: <cover.1291196789.git.amit.shah@redhat.com>
	<201012011159.35947.paul@codesourcery.com>
	<20101201121256.GD2962@amit-x200.redhat.com>
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To: Amit Shah <amit.shah@redhat.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.

Doesn't that cause the first partial chunk to be incorrectly transmitted 
twice? You may only return EAGAIN if no data was transmitted.

Paul