linux-kernel - Re: [PATCH v2 1/6] virtio_balloon: transitional interface

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Date:	Wed, 1 Apr 2015 12:57:48 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/6] virtio_balloon: transitional interface

On Wed, 1 Apr 2015 12:28:30 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Wed, Apr 01, 2015 at 12:22:44PM +0200, Cornelia Huck wrote:
> > On Wed, 1 Apr 2015 11:43:46 +0200
> > "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > 
> > > On Wed, Apr 01, 2015 at 07:53:14PM +1030, Rusty Russell wrote:
> > > > "Michael S. Tsirkin" <mst@...hat.com> writes:
> > > > > On Wed, Apr 01, 2015 at 02:17:23PM +1030, Rusty Russell wrote:
> > > > >> I would leave the device *exactly* as is, ugly structure packing and
> > > > >> all.
> > > > >
> > > > > But why?  It's going to be used for years, might as well make it clean?
> > > > 
> > > > Because the only spec which currently exists says to do that.
> > > 
> > > OK but the only spec which currently exists also says it's a legacy only
> > > device, so driver must not set VERSION_1.  So surely, we can make minor
> > > changes when VERSION_1 is set, like we did for other devices.
> > 
> > But we don't plan to replace the other devices, so it makes sense to do
> > some changes for 1.0.
> 
> I'm not sure what the above says. Do you agree with
> making minor changes in device behaviour?

The other way around.

> Also to be clear, I think this is 1.1 material.

Btw, I'd really like to see your proposed spec updates.

> 
> > > 
> > > Let me post the latest patches I'm working on,
> > > see what you think then.
> > > 
> > > >  We do
> > > > need a new virtio memballoon spec, but it'll look nothing like this
> > > > anyway.
> > > > 
> > > > Cheers,
> > > > Rusty.
> > > 
> > > I think it's going to have significantly different semantics, too,
> > > so not much value in making that one work with current
> > > drivers, right?
> > > 
> > 
> > So why not just keep virtio-balloon as-is and just specify endianness
> > etc. for 1.0? Keeps the old drivers going without hacks,
> > and we can
> > start with a fresh driver for the new virtio-balloon.
> 
> Well it doesn't really, we need cpu_to_virtio in a bunch of
> places anyway.

Of course, but what about keeping changes minimal?

> 
> So I kind of prefer making it clean, even just to avoid setting a bad
> example for other devices.
> 
> Let me post the new patch where it's all fixed in a cleaner way, and
> everyone can discuss whether it's too much work.
> 

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