Re: [PATCH v2 0/6] target-i386: Make most CPU models work with "enforce" out of the box

Eduardo Habkost
2014-08-26T18:01:28Z

On Tue, Aug 26, 2014 at 02:56:21PM +0200, Paolo Bonzini wrote:
> Il 25/08/2014 22:45, Eduardo Habkost ha scritto:
> >
> > TCG users expect the default CPU model to contain most TCG-supported features
> > (and it makes sense). See, for example, commit
> > f1e00a9cf326acc1f2386a72525af8859852e1df.
>
> It doesn't though (SMAP is the most egregious omission, and probably the
> main reason why people use QEMU TCG these days), and it raises the
> question of backwards-compatibility of qemu64---should we disable TCG
> features in old machine types? Probably yes, but we've never done that.

Had we changed qemu64, any changes to the feature set of qemu64 would
probably require compatibility code on old machine-types for KVM,
anyway. But the last time qemu64 was changed was in 2009 (commit
f1e00a9cf326acc1f2386a72525af8859852e1df), it looks like everybody was
afraid of touching "qemu64" because its purpose was not very clear.

So maybe that's good news, as things can be simpler if we make both TCG
and KVM have similar behavior:

* qemu64: a conservative default that should work out of the box on
most systems, for both TCG and KVM. That's already the current status,
we just need to document it.
* -cpu host: for people who want every possible feature to be enabled
(but without cross-version live-migration support). We can easily add
support for "-cpu host" to TCG, too.

--
Eduardo