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Re: [Xen-devel] [PATCH] Remove hardcoded xen-platform device initialization

On Fri, 14 Jun 2013, Paul Durrant wrote:
> > -----Original Message-----
> > From: Paolo Bonzini [mailto:paolo.bonzini@xxxxxxxxx] On Behalf Of Paolo
> > Bonzini
> > Sent: 14 June 2013 15:58
> > To: Paul Durrant
> > Cc: Ian Campbell; Stefano Stabellini; qemu-devel@xxxxxxxxxx; xen-
> > devel@xxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH] Remove hardcoded xen-platform device
> > initialization
> > 
> > Il 14/06/2013 10:11, Paul Durrant ha scritto:
> > > I think we're still going to need -M xenpv, I think; it's quite
> > > distinct from pc.
> > 
> > Of course!  Even more: "-M xenpv" should be reused on ARM.
> > 
> > > I guess we could use -M pc for HVM and gate the
> > > accel code as you suggest but, if that's the way we're going, it
> > > would seem more logical just to ditch the accel code for xenpv
> > > completely (assuming we can do all we need from the machine init) and
> > > then use -M pc -accel=xen for HVM guests going forward.
> > 
> > There is common code between pv and fv, and that one definitely belongs
> > in xen_init.  Most fv-only code probably should be in pc_init.  The rest
> > should move to xen_init though, because it would apply just as well for
> > example to Q35.  It's a bit ugly to have fv-only code there, but it's
> > better than having a Xen-specific machine type.  Xen/KVM/TCG should be
> > as similar as possible at the QEMU level, any difference should be
> > handled in the toolstack.
> > 
> > > But that does
> > > rather screw up my autodiscovery plans because I would not know, for
> > > a given qemu binary, which machine type to use.
> > 
> > There's no need for that.  4.4 can just use "-M pc" unconditionally,
> > <=4.3 will just use "-M xenfv" unconditionally.
> > 
> > > If I create a new
> > > xenfv-2.0 machine type though I *can* do auto discovery... in which
> > > case do we need the -accel=xen option at all?
> > 
> > Yes.  Please try not do things differently from other accelerators.
> > 
> Ok. I guess we can have the ability to override the machine type in the VM 
> config, so you could still kick off an older qemu with a newer libxl - but it 
> sounds like the auto-discovery idea is a no-go then.

xenfv-2.0 is a bad idea, like Paolo wrote, it should be possible to just
use -M pc for HVM guests and retain -M xenpv for pv guests.

However it seems to me that we also need a way in libxl to find out
whether QEMU is new enough for us to be able to use -M pc.
We can't just assume that users will be able to figure out the magic
rune they need to write in the VM config file to solve their VM crash at
boot problem.

We could spawn an instance of QEMU just to figure out the QEMU version
but we certainly cannot do that every time we start a new VM.
Once we figure out the QEMU version the first time we could write it to
xenstore so that the next time we don't have to go through the same
process again.

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