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Re: [Xen-devel] Multiple platform PCI device ID registries?



> -----Original Message-----
> From: Ian Campbell
> Sent: 13 November 2013 13:14
> To: Paul Durrant
> Cc: xen-devel; Ian Jackson; Stefano Stabellini
> Subject: Re: Multiple platform PCI device ID registries?
> 
> On Wed, 2013-11-13 at 12:50 +0000, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Ian Campbell
> > > Sent: 13 November 2013 12:39
> > > To: Paul Durrant
> > > Cc: xen-devel; Ian Jackson; Stefano Stabellini
> > > Subject: Re: Multiple platform PCI device ID registries?
> > >
> > > On Wed, 2013-11-13 at 12:30 +0000, Paul Durrant wrote:
> > > > > -----Original Message-----
> > > > > From: Ian Campbell
> > > > > Sent: 13 November 2013 12:09
> > > > > To: Paul Durrant
> > > > > Cc: xen-devel; Ian Jackson; Stefano Stabellini
> > > > > Subject: Re: Multiple platform PCI device ID registries?
> > > > >
> > > > > On Wed, 2013-11-13 at 11:58 +0000, Paul Durrant wrote:
> > > > >
> > > > > > > I'm concerned because that comment refers to XenServer 6.1 but
> it
> > > now
> > > > > > > appears to be being reused as the default device ID for the "Xen
> > > > > > > pvdevice".
> > > > > > >
> > > > > > > Maybe it is safe to reuse this in this way, but the docs should be
> > > > > > > updated I think.
> > > > > > >
> > > > > >
> > > > > > I believe it is safe.
> > > > >
> > > > > How would you describe it in that document? "Xen PV device
> (extended
> > > > > platform device). Previously used in XenServer 6.1" ?
> > > > >
> > > > > Is there some spec we can link to regarding how this device should be
> > > > > used?
> > >
> > > ???
> >
> > Sorry, missed that.
> 
> You've missed the part about a link to a spec for what the Xen PV device
> is, when it should appear, how it relates to the platform device again

Oh sorry - too many emails in different threads... No, there is no spec. in 
existence. The xl.cfg manpage refers readers to pci-device-reservations.txt so 
I guess that would be the best place to add some words.

> 
> > No, it wasn't previously used in 6.1 because it's not really the same
> > (as the pv device doesn't implement the fixed IO ports). I also don't
> > want to confuse it with the platform device, for that reason. It is a
> > new distinct and I agree that the fact it defaults to device ID is
> > confusing although it's safe - I will therefore submit a patch to QEMU
> > to modify it as I suggested before, so that the id *must* be specified
> > by the toolstack.
> 
> Ack.
> 
> > >
> > > Best bet is to just document "Don't Do That Then". Is there some
> > > XenServer way we can tell people to fix this (by changing the ID back?)
> > >
> >
> > Well part of the problem is that we don't support use of PVonHVM linux
> > in XenServer at all! The best thing is probably to tackle this on the
> > XenServer forums if and when someone posts the problem there. Adding
> > the extra blacklisting code to XenServer's QEMU and then getting that
> > into a hotfix should hopefully avoid future problems too - although we
> > may get 'why are my PV frontends not working?'-type questions.
> 
> Ah, I see now why it is a XS side fix.
> 
> > > I think "no platform device and no pv device" is a valid and useful
> > > configuration, meaning "use emulated devices". "no platform device, yes
> > > pv device" is the one to avoid.
> > >
> >
> > Hmm. How about splitting out the fixed IO ports then? That way the
> > platform device could be safely turned off if the PV device was
> > present. It seems like a cleaner and safer thing to do.
> 
> I thought when the PV device was added it was agreed that it would only
> ever be as an extension to the platform device, not a replacement for
> it.
> 
> Otherwise you get into situations where cloud providers need to know
> which to provide, whereas with a baseline platform device always there
> things can try and work.
> 

Yes, agreed, it is intended as an extra device but the fact that removing the 
*PCI* platform device from the VM disables the fixed IO ports is somewhat 
counter-intuitive so I was proposing that should be fixed. Then *if* someone 
had the PV device and removed the platform device any bound PV frontends would 
continue to function.

  Paul

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