Re: [Qemu-devel] [libvirt] [PATCH v3 2/2] Add configure option --enable-pc-1-0-qemu-kvm

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 22, 2014 at 05:24:55PM +0200, Markus Armbruster wrote:
> "Daniel P. Berrange" <berrange@redhat.com> writes:
> 
> > On Mon, Sep 22, 2014 at 02:36:55PM +0300, Michael S. Tsirkin wrote:
> >> On Sun, Sep 21, 2014 at 03:38:59PM +0100, Alex Bligh wrote:
> >> > Add a configure option --enable-pc-1-0-qemu-kvm and the
> >> > corresponding --disable-pc-1-0-qemu-kvm, defaulting
> >> > to disabled.
> >> > 
> >> > Rename machine type pc-1.0 to pc-1.0-qemu-git.
> >> > 
> >> > Make pc-1.0 machine type an alias of either pc-1.0-qemu-kvm
> >> > or pc-1.0-qemu-git depending on the value of the config
> >> > option.
> >> > 
> >> > Signed-off-by: Alex Bligh <alex@alex.org.uk>
> >> 
> >> I have to say, this one bothers me.
> >> We end up not being able to predict what does pc-1.0
> >> reference.
> >
> > Yeah, this is not good. Any single machine type name should have
> > fixed semantics - having two different semantics depending on build
> > options means mgmt apps can no longer simply compare the machine
> > type name to determine if it is a match with the same name on a
> > different host.
> 
> You're right.  However, this particular horse left the barn a long time
> ago: the pc-* machine types differ in qemu-kvm and upstream QEMU.
> 
> Sure, when qemu-kvm was merged back into QEMU, its machine type variants
> were dropped.  But they live on in various downstreams that just like
> QEMU had to pick between compatibility with upstream QEMU and qemu-kvm,
> but unlike QEMU picked compatibility with qemu-kvm.
> 
> So this patch does *not* break any management apps by letting them "no
> longer simply compare the machine type name to determine if it is a
> match with the same name on a different host".  They never could for
> these messed up machine types, at least not without knowing exactly what
> kind of QEMU runs on the hosts in question.
> 
> All this patch does is adding another facet to "exactly what kind of
> QEMU".

Right, but IMHO doing it at compile-time is wrong.
If distros want compatiblity with both sometimes, what then?
Build two binaries with different flags?
Should be a runtime option that management sets after (somehow?)
figuring out what's going on, on source.

How does it do that? Not sure - but I'm sure destination distro has no
way to figure it out.

-- 
MST