Re: [Qemu-devel] [PATCH 0/2] target-i386: "custom" CPU model + script to dump existing CPU models

[Eduardo Habkost]

On Tue, Jun 23, 2015 at 06:44:57PM +0200, Andreas Färber wrote:
> Am 23.06.2015 um 18:38 schrieb Eduardo Habkost:
> > On Tue, Jun 23, 2015 at 06:33:05PM +0200, Michael S. Tsirkin wrote:
> >> On Tue, Jun 23, 2015 at 05:25:55PM +0100, Daniel P. Berrange wrote:
> >>> Whether QEMU changed the CPU for existing machines, or only for new
> >>> machines is actually not the core problem. Even if we only changed
> >>> the CPU in new machines that would still be an unsatisfactory situation
> >>> because we want to be able to be able to access different versions of
> >>> the CPU without the machine type changing, and access different versions
> >>> of the machine type, without the CPU changing. IOW it is the fact that the
> >>> changes in CPU are tied to changes in machine type that is the core
> >>> problem.
> >>
> >> But that's because we are fixing bugs.  If CPU X used to work on
> >> hardware Y in machine type A and stopped in machine type B, this is
> >> because we have determined that it's the right thing to do for the
> >> guests and the users. We don't break stuff just for fun.
> >> Why do you want to bring back the bugs we fixed?
> > 
> > I didn't take the time to count them, but I bet most of the commits I
> > listed on my previous e-mail message are not bug fixes, but new
> > features.
> 
> Huh? Of course the latest machine model get new features. The point is
> that the previous ones don't and that's what we are providing them for -
> libvirt is expected to choose one machine and the contract with QEMU is
> that for that machine the CPU does *not* grow new features, and we're
> going at great lengths to achieve that. So this thread feels more and
> more weird...

We are not talking about changes to existing machines. We are talking
about having changes introduced in new machines (the one we did on
purpose) affecting the runnability of the VM.

-- 
Eduardo