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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-11-13 at 13:25 +0000, Paul Durrant wrote:
> > -----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.

Do you think you could do that? I'm clearly to confused to write
anything useful...

> 
> > 
> > > 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.

If someone removes the platform device then this should cause the PV
device to be removed or for configurations which specify PV but not
platform device to be rejected.

Ian.


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