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

[Stefano Stabellini]

On Tue, 18 Jun 2013, Paolo Bonzini wrote:
> Il 18/06/2013 20:56, Stefano Stabellini ha scritto:
> >> > 
> >> > 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.
> 
> Can you just assume that 4.4 requires QEMU 1.6 or newer?

I would rather not make that assumption because we cannot control what
distro are going to package. I wouldn't want a distro to ship with Xen
HVM guests broken because they choose the wrong QEMU version. Of course
we could put that in the release notes, but there are lots of distros
out there and I am pretty sure that at least one of them is not going to
read them.