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

[Daniel P. Berrange]

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"/>

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.

Regards,
Daniel
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