Re: [PATCH 00/10] PV-IO v3

Rusty Russell
2007-08-20T23:24:46Z

On Mon, 2007-08-20 at 17:12 +0300, Avi Kivity wrote:
> Dor Laor wrote:
> > Using Rusty's code there is no waste.
> > Each descriptor has a flag (head|next). Next flag stands for pointer to
> > the
> > next descriptor with u32 next index. So the waste is 4 bytes.
> > Sg descriptors are chained on the same descriptor ring.
> >
>
> Block I/O can easily require 256 sglist entries, eating up your ring.

Absolutely; I think we'd want this variable sized, rather than a single
page as the current implementation does for no really sound reason.

That said, I don't have a problem with Avi's out-of-band mechanism. A
"DMA" hypercall can follow that almost as easily, and it does give that
nice "fixed number of slots" effect.

Cheers,
Rusty.