Re: [Qemu-devel] [Xen-devel] [PATCH] Add Xen platform PCI device version 2.

[Ian Campbell]

On Thu, 2013-06-20 at 09:56 +0100, Tim Deegan wrote:
> At 07:47 +0000 on 20 Jun (1371714432), Paul Durrant wrote:
> > > > I agree. If this is really the only solution, we would need to have
> > > > both versions presented to the guest so that old drivers continue to
> > > > work without any intervention.
> > > 
> > > I suspect that if we expose both, both sets of drivers try to run the
> > > same PV connections, and hilarity ensues.
> > > 
> > 
> > Actually I think I can make that work, and it is the conclusion I came
> > to after Alex's comment.
> 
> Ah, nice!  In that case, I'm a lot less worried -- we can just expose
> both versions/devices by default and there's no need for a visible
> control knob tied to driver version (except maybe for debugging).
> 
> It means an 'unsupported' device appearing on other/older OSes, which is
> unfortunate, but ISTR only Windows really complains visibly about that.

I'm not at all convinced this is a good approach. Are we going to add a
third, forth and fifth device whenever Linux, BSD, $other-OS paint
themselves into a corner somehow WRT their internal driver model vs
their Xen PV drivers?

AFAIK the Citrix PV drivers have never been formally supported on
anything other than XenServer and XCP (and I'm not sure about "formally"
for XCP), so this is really an issue of supporting upgrades for people
running those. I think rather than making hacks upstream, which will
effectively need to be supported forever, the hack should be done on the
XenServer side and take advantage of whatever the supported upgrade path
is (N+1 or N+2 or whatever). This way the hack can eventually go away.
For anyone who grabbed the older drivers and used them outside of the
context of XenServer or XCP this is a documentation/awareness issue.

Can we use the blacklisting functionality of the PV unplug protocol to
blacklist previous versions of the Citrix PV drivers? I wouldn't
consider this an unsuitable thing to do in upstream, in fact it would be
using it for exactly the purpose for which it was designed. As long as
this is sufficient to boot with emulated devices in order to switch to
the newer drivers that should be good enough.

Ian.