Re: [Qemu-devel] [PATCH v8 7/7] virtio-console: Enable port throttling when chardev is slow to consume data

[Amit Shah]

On (Wed) Dec 01 2010 [13:08:26], Paul Brook wrote:
> > 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.

Except for the fact that no caller of qemu_chr_write() resubmits (or
even checks) partial writes.

I think the only way to solve this in a caller-neutral way while keeping
the current semantics is to buffer all the data that comes in for
qemu_chr_write() and re-submit partial writes in the unblock routine
(ie, what I did in virtio-console.c in v7 is to be done in qemu-char.c
now).

That OK?

                Amit