On Wed, 2020-11-04 at 18:04 +0000, Daniel P. Berrangé wrote:
On Wed, Nov 04, 2020 at 07:00:16PM +0100, Dario Faggioli wrote:
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."
Right so it is basically a hack to workaround problem with historical defaults in QEMU. In the libvirt case we're not dealing with defaults, we have explicit XML element to express what we want. So we can express it straight away and not need this hack.
Ok.
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)
[...]
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?
I don't see any functional difference between 1 & 2 from libvirt's side. In fact (2) is probably better because it can work on any version of QEMU, even before host-phys-bits-limit was introduced.
That is indeed true.
The "downside" is that an app has to read the capabilities to see the current host limit and choose the "NN" value.
That's why I happen to think there could be value in having limit... Or does this all just not make sense? :-)
I think we can ignore host-phys-bits-limit. If it later turns out we really do need it, we can add it to libvirt, but until then pretend it doesn't exist.
Right. I think I've understood your line of reasoning and I agree. Especially on the fact that we can ignore it for now, and always add it later, if we see the need for it. And I certainly can look into adding the phys bits to the host capabilities. Regards -- Dario Faggioli, Ph.D http://about.me/dario.faggioli Virtualization Software Engineer SUSE Labs, SUSE https://www.suse.com/ ------------------------------------------------------------------- <<This happens because _I_ choose it to happen!>> (Raistlin Majere)