Re: [Qemu-devel] [PATCH x86-next v2] target-i386: add PCID flag to Westmere, Sandy Bridge and Ivy Bridge

[Kashyap Chamarthy]

On Tue, Jan 16, 2018 at 01:55:22PM +0100, Vincent Bernat wrote:
>  ❦ 16 janvier 2018 10:41 -0200, Eduardo Habkost <address@hidden> :
> 
> >> > Adding Westmere-PCID would require adding a Westmere-PCID-IBRS
> >> > CPU model too, so this is starting to look a bit ridiculous.
> >> > Sane VM management systems would know how to use
> >> > "-cpu Westmere,+pcid" without requiring new CPU model entries in
> >> > QEMU.  What's missing in existing management stacks to allow that
> >> > to happen?
> >> 
> >> That's what I actually do. So, I am fine with the solution of doing
> >> nothing. However, it would be nice for unaware people to get the speedup
> >> of pcid without knowing about it. Maybe we can just forget about
> >> Westmere and still apply it to Sandy Bridge and Ivy Bridge.
> >
> > If management stacks today don't let the user choose
> > "Westmere,+pcid", we probably have no other choice than adding a
> > Westmere-PCID CPU model.  But our management stacks need to be
> > fixed so we won't need similar hacks in the future.

True;  I'm aware of the limitation here in Nova.

> With libvirt:
> 
>   <cpu mode='custom' match='exact'>
>     <model>Westmere</model>
>     <feature policy='require' name='pcid'/>
>   </cpu>

Yep, libvirt upstream allows it.

> We are using CloudStack on top of that and it's also an available
> option. However, looking at OpenStack, it doesn't seem possible:
>  
> https://github.com/openstack/nova/blob/6b248518da794a4c82665c22abf7bee5aa527a47/nova/conf/libvirt.py#L506

That's correct, upstream OpenStack Nova doesn't yet have facility to
specify granular CPU feature names.  Nova just ought to wire up the
facility libvirt already provides.

-- 
/kashyap