Re: [PATCH 12/28] virtio: console: Buffer data that comes in from the host

[Amit Shah <amit.shah@redhat.com>]

On (Wed) Dec 02 2009 [14:14:20], Rusty Russell wrote:
> On Sat, 28 Nov 2009 05:20:35 pm Amit Shah wrote:
> > The console could be flooded with data from the host; handle
> > this situation by buffering the data.
> 
> All this complexity makes me really wonder if we should just
> have the host say the max # ports it will ever use, and just do this
> really dumbly.  Yes, it's a limitation, but it'd be much simpler.

As in make sure the max nr ports is less than 255 and have per-port vqs?
And then the buffering will be done inside the vqs themselves?

> > --- a/drivers/char/virtio_console.c
> > +++ b/drivers/char/virtio_console.c
> > @@ -65,6 +65,23 @@ struct ports_device {
> >  	 * interrupt
> >  	 */
> >  	struct work_struct rx_work;
> > +
> > +	struct list_head unused_read_head;
> 
> You should name lists after plurals, rather than using "head" which is
> an implementation detail.  eg. "queued_inbufs" and below "used_inbufs".

OK.

> Though Shirly Ma was working on a "destroy_bufs" patch which would avoid
> your need for this list at all, AFAICT.
> 
> > +		/* Return the number of bytes actually copied */
> > +		ret = copy_size;
> > +		buf->offset += ret;
> > +		out_offset += ret;
> > +		out_count -= ret;
> 
> We don't actually use ret.

In a later patch, when copy_to_user is added, ret will be used. So I
kept it this way to reduce the noise in the diffs later.

> > +		if (buf->len - buf->offset == 0) {
> 
> I prefer the simpler "if (buf->offset == buf->len)" myself.

Will update.

> > +			spin_lock_irqsave(&port->readbuf_list_lock, flags);
> > +			list_del(&buf->list);
> > +			spin_unlock_irqrestore(&port->readbuf_list_lock, flags);
> > +			kfree(buf->buf);
> > +			kfree(buf);
> 
> Does it become cleaner later to have this in a separate function?  Usually
> I prefer matching alloc and free fns.

Adding a function is easy, sure. I should've done that though; something
that got overlooked.

> > +static struct port_buffer *get_buf(size_t buf_size)
> > +{
> > +	struct port_buffer *buf;
> > +
> > +	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
> > +	if (!buf)
> > +		goto out;
> > +	buf->buf = kzalloc(buf_size, GFP_KERNEL);
> > +	if (!buf->buf) {
> > +		kfree(buf);
> > +		goto out;
> 
> No, that would return non-NULL.  I'd stick with the standard multi-part exit:
> 
> 	if (!buf)
> 		goto fail;
> 	buf->buf = kzalloc(buf_size, GFP_KERNEL);
> 	if (!buf->buf)
> 		goto fail_free_buf;
> 	buf->len = buf_size;
> 	return buf;
> 
> fail_free_buf:
> 	kfree(buf);
> fail:
> 	return NULL;

Ow, indeed.

Thanks! 

		Amit