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

[Ian Campbell]

On Tue, 2013-06-18 at 23:38 +0200, Andreas Färber wrote:
> Am 18.06.2013 21:35, schrieb Stefano Stabellini:
> > On Tue, 18 Jun 2013, Paolo Bonzini wrote:
> >> Il 18/06/2013 20:56, Stefano Stabellini ha scritto:
> >>> 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.
> 
> You could check for existence of the pc-i440fx-1.6 machine and infer
> that it is at least v1.6 (might break in some distant future of course
> and for current git commits until your changes get merged).

Actually, this raises an interesting point. AIUI "pc" is simply and
alias for the most recent "pc-X.Y" and "pc-X.Y" is present to allow for
qemu "upgrading" the set of emulated hardware, as in it represents
changing the set of emulated peripherals, not just fixing bugs in the
emulation etc, is that right?

I think it would be preferable for us to request a specific platform
(pc-i440fx-1.6 if that's the one) and a conscious decision to support
newer platforms (and can test it etc).

Ian.