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

[Jiri Denemark]

On Thu, Apr 20, 2017 at 11:07:27 +0100, Daniel P. Berrange wrote:
> On Thu, Apr 20, 2017 at 11:53:21AM +0200, Jiri Denemark wrote:
> > 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.
> 
> I don't think there's any dependancy on -cpu host actually.
> 
> Semantically host-cache-info=on, only makes sense if the sockets/core/threads
> topology of the guest, matches the host. What other CPUID bits you choose is
> tangential.

QEMU currently supports host-cache-info only for -cpu host. "Property
'.host-cache-info' not found" if you try with any other CPU model. And
device-list-properties QMP command confirms that, only host-x86_64-cpu
type lists host-cache-info property. We'd probably need to check whether
the property is available for the specific CPU model :(

Jirka