Re: [PATCH 18/31] virtio: console: Buffer data that comes from the host

[Rusty Russell <rusty@rustcorp.com.au>]

On Thu, 14 Jan 2010 01:47:53 pm Amit Shah wrote:
> On (Thu) Jan 14 2010 [09:45:12], Rusty Russell wrote:
> > On Wed, 13 Jan 2010 09:54:52 pm Amit Shah wrote:
> > > On (Wed) Jan 13 2010 [21:43:32], Rusty Russell wrote:
> > > > On Wed, 13 Jan 2010 04:41:48 pm Amit Shah wrote:
> > > > > On (Mon) Jan 04 2010 [15:17:17], Amit Shah wrote:
> > > > > > On (Mon) Jan 04 2010 [19:45:30], Rusty Russell wrote:
> > > > > > > On Wed, 23 Dec 2009 01:04:28 am Amit Shah wrote:
> > > > > > > > The console could be flooded with data from the host; handle this
> > > > > > > > situation by buffering the data.
> > > > > > > 
> > > > > > > Is this still true?  If we only add_buf when we're ready, surely the host
> > > > > > > can't flood us with one virtqueue per port?
> > > > > > 
> > > > > > I guess I meant something completely different. This message is
> > > > > > definitely misleading and I'll re-word it.
> > > > > > 
> > > > > > You're right; we don't need the 'guest throttling' feature that was
> > > > > > needed earlier.
> > > > > 
> > > > > BTW I meant this series doesn't have the guest throttling feature.
> > > > > 
> > > > > Rusty, did you just have this comment for the series? If yes, I'll just
> > > > > re-send this patch with a fixed description.
> > > > 
> > > > I don't see why we ever allocate more than one incoming buffer though?
> > > 
> > > To prevent against a fast host app sending data to a slow guest
> > > consumer.
> > > 
> > > Also, we use the in_vq for the buffering, so the number of buffers is
> > > limited by the queue size that's declared by the host.
> > 
> > But if the guest only ever registers one input buffer at a time, we get the
> > same effect.  And it means we use less memory.  And our code is simpler.
> 
> The number of buffers to be registered in the vq comes from the host
> anyway; if the host wants to be able to use multiple buffers, we should
> be able to use them.

I have sympathy with this, but I'd prefer to see the minimal solution first
and look at this later.  A single buffer should make for simpler code...

> Also, we lose out on the ability of the host to send us data when the
> guest is slow to read.

That happens at some point anyway.

Thanks,
Rusty.