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

[Paolo Bonzini]

Il 26/08/2014 20:01, Eduardo Habkost ha scritto:
> 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.

This means that "-cpu host" has different meanings in KVM and TCG.  Is
that an advantage or a disadvantage?

If I have to choose blindly, I'd rather give different (but sane)
meanings to "-cpu qemu64" and the same meanings to "-cpu host"...
Basically "-cpu qemu32/64" on KVM would be changed automatically to
kvm32/64.

Paolo