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

[Michael S. Tsirkin]

On Wed, Jun 24, 2015 at 04:35:13PM +0200, Andreas Färber wrote:
> Am 24.06.2015 um 16:19 schrieb Michael S. Tsirkin:
> > On Wed, Jun 24, 2015 at 11:16:51AM -0300, Eduardo Habkost wrote:
> >>> IMHO -M pc is not supposed to mean "can break at any time".
> >>
> >> It means "it may have new host-side requirements and may become runnable
> >> in your host (or require additional command-line flags to run) at any 
> >> time".
> > 
> > That would be pretty bad. I don't think we ever had such cases in practice.
> 
> Why is that bad or unexpected? If you install new software, it may have
> new dependencies. QEMU would be no different from other software there.

That's just basic compatibility.  If I run using same flags, I expect
compatible behaviour.

How would you like it if each time you update bash, all your scripts had
to stop working, unless you specify a specific compability flag in
scripts, in which case you miss (some) bugfixes?

Or if you found code snippets and they can't both work.  Why? Because
snippet 1 says request version 2.4 compatibility and another says use version 
2.5
compatibility, so you can't use both.

Each time you break command line compatibility, you
invalidate an unknown chunk of useful documentation somewhere.


> Yesterday Eduardo said it was about having a fixed version installed and
> therein switching from a legacy machine to a newer machine. In that case
> the dependencies existed ever since installation but were not
> immediately visible to the end user due to our stability guarantees.
> 
> Andreas
> 
> -- 
> SUSE Linux GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
> GF: Felix Imendörffer, Jane Smithard, Dilip Upmanyu, Graham Norton; HRB
> 21284 (AG Nürnberg)