On Wed, Oct 03, 2012 at 04:14:17PM +0930, Rusty Russell wrote:

"Michael S. Tsirkin" [off-list ref] writes:

quoted

Thinking about Sasha's patches, we can reduce ring usage
for virtio net small packets dramatically if we put
virtio net header inline with the data.
This can be done for free in case guest net stack allocated
extra head room for the packet, and I don't see
why would this have any downsides.

I've been wanting to do this for the longest time... but...

quoted

Even though with my recent patches qemu
no longer requires header to be the first s/g element,
we need a new feature bit to detect this.
A trivial qemu patch will be sent separately.

There's a reason I haven't done this.  I really, really dislike "my
implemention isn't broken" feature bits.  We could have an infinite
number of them, for each bug in each device.

So my plan was to tie this assumption to the new PCI layout.

I don't object but old qemu has this limitation for s390 as well,
and that's not using PCI, right? So how do we detect
new hypervisor there?

-- 
MST