[PATCH v1] qemu: monitor: substitute missing model name for host-passthrough

[Jiri Denemark]

Collin, I apologize for not getting to you earlier.

On Wed, Sep 16, 2020 at 12:11:08 -0400, Collin Walling wrote:
> On 9/16/20 3:03 AM, Michal Privoznik wrote:
> > On 9/15/20 10:25 PM, Collin Walling wrote:
> >> One more ping in attempt to get this in the right direction. Otherwise
> >> I'll post my next idea and we can go from there :)
> > 
> > I agree with Peter that while the idea might look correct it's too deep.
> > 
> >>
> >> Thinking about this issue, should a host-passthough CPU definition be
> >> permitted for the baseline & comparison commands? The model represented
> >> under this mode is not considered migration safe and it may make sense
> >> to simply fail early since these commands aim to construct/determine a
> >> migratable CPU model, respectively.
> > 
> > Honestly, I don't know much about this CPU models area, but is that true
> > even for two identical hosts? Say I have two desktops next to each
> > other, with the same CPU and I want to migrate. I could use host model,
> > couldn't I?
> > 
> 
> "Host-model" is an alias for a CPU model that closely represents the
> capabilities of the host machine (on s390, because this model is defined
> by the hypervisor, it can also be called the "hypervisor CPU model" --
> not an important detail).
> 
> However, a guest running with the host-passthrough mode is not
> considered migration safe as that guest may covertly run with
> features/capabilities that are not directly exposed to the hypervisor.

Right, but migration may still be possible and working fine if both host
are identical.

> From what I understand regarding the hypervisor-cpu-compare and
> hypervisor-cpu-baseline commands is that they aim to assist with
> determining the migratability of guests based on their CPU model and
> feature set (usually along with a host CPU in the equation as well).

Baseline with a host-passthrough CPU is not indeed very useful, but
compare could still be used and its usage is not limited to migration.
For example, you can use it to check whether a domain with a guest CPU
configuration can be started on a specific host before you actually try
to start it. And reporting host-passthrough as incompatible would be
wrong.

Anyway, thanks for your patch, it was mostly correct, it just needed to
be done a bit higher in the call graph. Incidentally, Tim Wiederhake [1]
took this original patch and moved the change to the right place. The
authorship is still yours, so if you want to append you signed-off-by
tag there, I'll wait a bit before pushing Tim's patch.

Jirka

[1] https://www.redhat.com/archives/libvir-list/2020-September/msg01177.html