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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Campbell
> Sent: 26 June 2013 11:40
> To: Tim (Xen.org)
> Cc: Paul Durrant; Matt Wilson; Alex Bligh; xen-devel@xxxxxxxxxxxxx; qemu-
> devel@xxxxxxxxxx
> Subject: Re: [Xen-devel] [Qemu-devel] [PATCH] Add Xen platform PCI device
> version 2.
> 
> 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?
> 

Is there any harm in having a separate device for each OS's PV drivers? I agree 
it's not entirely elegant, but at least it allows for revision control when you 
need it.

> 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.
> 

We could blacklist all existing Citrix PV drivers in upstream QEMU, to avoid 
the clash, but that seems like a very unfriendly approach. Also, it's not going 
to stop someone with an existing VM, who happens to be using legacy Citrix PV 
drivers (an AWS VM for instance) receiving a driver from Windows Update that 
will blue-screen their VM on next reboot. Hence the only way forward is to bind 
the new drivers to something new, that we can control, so we know what driver a 
VM is going to get from Windows Update. And we may indeed need to modify its 
revision in future so that we can retire old sets of PV drivers and replace 
them with new ones, but only for newer XenServer releases. Thus, I also propose 
to make the PCI revision of the new device a command line parameter.

  Paul
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