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



On 9/23/20 9:50 AM, Jiri Denemark wrote:
> 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

Agreed, but I think baseline would still benefit from the error catching
that is proposed in the CPU comparison patch (I continue the
conversation over on that thread).

> 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.

Thanks. Gave my sign-off.

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




-- 
Regards,
Collin

Stay safe and stay healthy


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