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

[Jiri Denemark]

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?

Thanks,

Jirka

[1] https://patchwork.kernel.org/patch/9308401/