Date:	Mon, 14 Nov 2011 08:59:02 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexey Kardashevskiy <aik@...abs.ru>,
	Amit Shah <amit.shah@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Krishna Kumar <krkumar2@...ibm.com>,
	Pawel Moll <pawel.moll@....com>,
	Wang Sheng-Hui <shhuiw@...il.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	avi@...hat.com, penberg@...nel.org
Subject: Re: [PATCHv2 RFC] virtio-spec: flexible configuration layout

On Sun, Nov 13, 2011 at 05:14:27PM +0200, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2011 at 02:54:31PM +1030, Rusty Russell wrote:
> > On Wed, 09 Nov 2011 22:57:28 +0200, Sasha Levin <levinsasha928@...il.com> wrote:
> > > On Wed, 2011-11-09 at 22:52 +0200, Michael S. Tsirkin wrote:
> > > > On Wed, Nov 09, 2011 at 10:24:47PM +0200, Sasha Levin wrote:
> > > > > 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.
> 
> Okay, but going forward, if we add more capabilities, we probably won't
> want to require them and fail to load if not there.  That's really why I
> wanted to make the failover ignore any capability separately - to make
> this future proof.  I'm not terribly fixated on this, it just seemed a
> bit more symmetrical to treat all capabilities in the same way. Hmm?
> 
> >     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.).
> 
> Just to clarify, as written in draft this is possible with the current
> spec proposal.  So I'm guessing there's some other motivation that you
> had in mind?
> 
> > (2) There's no huge win in keeping the same layout.  Let's make some
> >     cleanups.
> 
> About this last point - what cleanups do you have in mind?  Just move
> some registers around?  I guess we could put feature bits near each
> other, and move device status towards the end to avoid wasting 3 bytes.
> The win seems minimal, but the change does make legacy hypervisor
> support in guests more cumbersome, as we need to spread coditional code
> around instead of localizing it in the initialization path.
> 
> >    There are more users ahead of us then behind us (I
> >     hope!).
> 
> In that case isn't it safe to assume we'll find some uses
> for the reserved registers?
> 
> > But I think this is the right direction!
> > 
> > Thanks,
> > Rusty.

Note: waiting on response to the above before I tweak the
spec again.

-- 
MST
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