Re: INDIRECT and NEXT

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 23, 2009 at 02:01:23PM +1030, Rusty Russell wrote:
> On Thu, 22 Oct 2009 08:07:45 pm Michael S. Tsirkin wrote:
> > Imagine an indirect entry where NEXT bit is also set.
> 
> Hmm, so it's not obvious whether the kvm userspace code handles it
> correctly either.

You mean qemu? I think it doesn't, either: code there looks basically
like lguest.

> Want to hack something up to use NEXT + INDIRECT, then we can actually test
> it?  If it doesn't work, this will have to be a new feature bit.
> 
> Also, we have a limitation that you can't have more descriptors than the ring
> size, even with indirect, due to overzealous checks...

Yes ... so I wonder: do we want to fix all this and add a feature bit,
or wait until some guest actually wants to use such descriptors?
For vhost, I implemented INDIRECT without this limitation
since it looked neater to me to have a separate function for indirect
anyway. This is because direct virtqueues are virtually contigious,
so I can access them just by copy from user, but indirect
can be spread around and so I have to go through extra translations.

> Thanks,
> Rusty.