Re: [PATCH] target/i386: add VMX features to named CPU models

[Paolo Bonzini]

On 20/11/19 19:45, Eduardo Habkost wrote:
> On Wed, Nov 20, 2019 at 06:37:53PM +0100, Paolo Bonzini wrote:
>> This allows using "-cpu Haswell,+vmx", which we did not really want to
>> support in QEMU but was produced by Libvirt when using the "host-model"
>> CPU model.
> 
> I understand guest ABI compatibility is not a concern, but I
> don't remember how we guarantee it won't break by accident if
> somebody tries to live migrate a VM.

I'm not sure I understand the question, but I can answer the second part:

> What is supposed to happen today if trying to live migrate a VM
> using "-cpu Haswell,+vmx"?

Before 4.2: same guest ABI compatibility as "-cpu host".

4.2+: ABI compatibility is preserved, because each named CPU model can
be given a precise set of features that are matched against the host
(and are subject to check/enforce).

4.1->4.2: the ABI *should* be preserved if you're running "-cpu
SandyBridge,+vmx" on an actual Sandy Bridge, but some VMX features will
disappear after live migration if e.g. you're running "-cpu
SandyBridge,+vmx" on a Haswell.  Host-model should be fine though.

Paolo