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

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Jan 16, 2018 at 03:08:15PM -0200, Eduardo Habkost wrote:
> [CCing Daniel]
> 
> On Tue, Jan 16, 2018 at 04:33:00PM +0100, Kashyap Chamarthy wrote:
> > On Tue, Jan 16, 2018 at 01:55:22PM +0100, Vincent Bernat wrote:
> > >  ❦ 16 janvier 2018 10:41 -0200, Eduardo Habkost <ehabkost@redhat.com> :
> > > 
> > > >> > 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.
> 
> I still don't understand why OpenStack doesn't let users add or
> modify elements on the domain XML.  This isn't the first time I
> see this preventing users from fixing problems or optimizing
> their systems.
> 
> Is there a summary of the reasons behind this limitation
> somewhere?

Exposing ability to control every aspect of Libvirt XML is a non-goal of
Nova. A great many of the features require different modelling and/or
explicit handling by Nova to work well in the context of OpenStack's
architecture. The domain XML is automatically generated on the fly by
Nova based on the info it gets from various inputs, so there's nothing
that can be editted directly to add custom elements. The only way that
would allow modification is for Nova to send the XML it generates to
an external plugin script and read back modified XML. Historically Nova
did have alot of plugin points that allowed arbitrary admin hacks like
this, but they end up being a support burden in themselves, as they
end up being black boxes which change Nova behaviour in unpredictable
ways. Thus Nova has actually worked to remove as many of the plugins
as possible.

In this case there is a clear benefit to being able to add extra CPU
features, over the base named model. It is easy for Nova to wire this
up and it should do so as a priority.

Regards,
Daniel
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