[libvirt] [RFC] Add support for QEMU's l3-cache CPU property

[Jiri Denemark]

On Thu, Apr 20, 2017 at 10:02:38 +0100, Daniel P. Berrange wrote:
> On Tue, Apr 18, 2017 at 04:22:36PM +0200, Jiri Denemark wrote:
> > Hi,
> > 
> > Apparently, reporting a level 3 cache on a virtual CPU can dramatically
> > increase performance in some use cases [1]. The interesting part is that
> > l3-cache=on does not provide the real CPU cache data, it's just making
> > it up. Anyway, we should be able to enable this via libvirt. And since
> > there is another property which enables real CPU cache data to be passed
> > to a guest, I suggest the following /domain/cpu/cache element equivalent
> > to l3-cache=on:
> > 
> >     <cache level='3' mode='emulate'/>
> > 
> > If we need to add support for passing the real CPU cache data, we can do
> > that with
> > 
> >     <cache level='3' mode='passthrough'/>
> > 
> > or we can even make the level attribute optional and support
> > 
> >     <cache mode='passthrough'/>
> > 
> > Missing cache element means default behaviour of the hypervisor and we
> > can eventually add <cache mode='disable'/> to turn off passing any CPU
> > cache info to the guest.
> > 
> > But I think we should now focus only on <cache level='3' mode='emulate'/>
> > and leaving the rest for the future when we actually need it.
> > 
> > This is how a more complete example would look like:
> > 
> >     <cpu mode='custom' match='exact'>
> >       <model>Broadwell</model>
> >       <cache level='3' mode='emulate'/>
> >     </cpu>
> > 
> > And libvirt would translate it into -cpu Broadwell,l3-cache=on.
> > 
> > Do you have any thoughts about the XML schema or naming?
> 
> The second QEMU property 'host-cache-info' causes the guest to see
> the host cache topology. This affects L1, L2 and L3 caches all at
> once. 
> 
> We could allow use of '<cache>' without specifying a level. ie
> 
>    <cache mode="passthrough"/>

Yes, this is what I suggested a few lines above. However, it seems
host-cache-info is only supported for -cpu host.

> to indicate passthrough of L1,L2 & L3 cache all together, mapping
> to host-cache-info=on. and the <cache level=3 mode=emulate>
> mapping to the l3-cache=on.
> 
> These two elements would need to be mutually exclusive.

Mixing them could allow host cache info to be passed through with
specific level-N cache data overridden by additional <cache level='N'/>
element. QEMU does not complain of both host-cache-info and l3-cache are
enabled. But anyway, making them mutually exclusive is probably the
safest option. However, what would happen if

    <cpu mode='host-passthrough'>
      <cache level='3' mode='emulate'/>
    </cpu>

is specified in domain XML?

    -cpu host,l3-cache=on

or

    -cpu host,host-cache-info=off,l3-cache=on

And similarly, I guess <cache mode="passthrough"/> should just
unconditionally disable l3-cache if we want to be sure they are always
mutually exclusive.

Jirka