Re: [PATCHv2 RFC] virtio-spec: flexible configuration layout

On Fri, Nov 11, 2011 at 02:54:31PM +1030, Rusty Russell wrote:

On Wed, 09 Nov 2011 22:57:28 +0200, Sasha Levin [off-list ref] wrote:

quoted

On Wed, 2011-11-09 at 22:52 +0200, Michael S. Tsirkin wrote:

quoted

On Wed, Nov 09, 2011 at 10:24:47PM +0200, Sasha Levin wrote:

quoted

It'll be a bit harder deprecating it in the future.

Harder than ... what ?

Harder than allowing devices not to present it at all if new layout
config is used. Right now the simple implementation is to use MMIO for
config and device specific, and let it fallback to legacy for ISR and
notifications (and therefore, this is probably how everybody will
implement it), which means that when you do want to deprecate legacy,
there will be extra work to be done then, instead of doing it now.

Indeed, I'd like to see two changes to your proposal:

(1) It should be all or nothing.  If a driver can find the virtio header
    capability, it should only use the capabilties.  Otherwise, it
    should fall back to legacy.  Your draft suggests a mix is possible;
    I prefer a clean failure (ie. one day don't present a BAR 0 *at
    all*, so ancient drivers just fail to load.).
(2) There's no huge win in keeping the same layout.  Let's make some
    cleanups.  There are more users ahead of us then behind us (I
    hope!).
But I think this is the right direction!

Thanks,
Rusty.

I'll do these changes, thanks!