[RFC PATCH 2/2] qemu: Add support for max physical address size

[Dario Faggioli]

On Wed, 2020-11-04 at 13:11 +0000, Daniel P. Berrangé wrote:
> On Mon, Nov 02, 2020 at 09:14:22AM +0100, Christian Ehrhardt wrote:
> > 
> > Looking at my todo notes I wondered if while touching it we should
> > right
> > away also
> > add host-phys-bits-limit in the same spot?
> > See https://git.qemu.org/?p=qemu.git;a=commit;h=258fe08bd341d
> 
> I'm not convinced we actally need it in libvirt. Assuming we report
> the
> host phys bits in the capabilities XML, apps can already achieve this
> functionality with the mode=emulate bits=NN settings  by populating
> NN
> based on the capabilities XML.
> 
Right. So, host-phys-bits-limit seems to exist because:
"Some downstream distributions of QEMU set host-phys-bits=on by
default. [...] Now choosing a large phys-bits value for a VM has bigger
impact: it will make KVM use 5-level EPT even when it's not really
necessary."

And we want to be able to, from QEMU: "keep using the host phys-bits
value, but only if it's smaller than 48."

So, I was thinking that, in a world where QEMU will
have host-phys-bits=on by default, if using `-cpu host` you may want to
be able to do the same from libvirt, i.e., "same as host, but not more
than NN".

In my head (and drafts), that would happen with:

<maxphysaddr mode="passthrough" limit="NN"/>   (1)

Which is very similar and may be identical to:

<maxphysaddr mode="emulate" bits="NN"/>        (2)

Whether or not they're actually identical, e.g., if NN > MM, depends on
what QEMU does. In that case, with (1) we set  MM. With (2), if QEMU
let us do that, we set NN. (And when I say what QEMU does, I don't
mean, right know, I mean in the particular version in use, assuming
that at least in theory it can change between different versions).

So, maybe having (1) may be the only way of making sure that I get
min(NN,MM), irrespective of QEMU version/behavior. Or am I missing
something?

That's why I happen to think there could be value in having limit... Or
does this all just not make sense? :-)

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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