[libvirt] [PATCH v4 0/8] CPU Model Baseline and Comparison for s390x

[David Hildenbrand]

On 10.09.19 17:22, Jiri Denemark wrote:
> On Tue, Sep 03, 2019 at 15:32:34 -0400, Collin Walling wrote:
>> On 8/20/19 10:06 AM, Jiri Denemark wrote:
>>> First, let me apologize for such a late review. I'll try my best to
>>> review your series earlier next time.
>>>
>>
>> Your review is greatly appreciated! I haven't replied to your other
>> posts on this series as my comments were mostly acknowledgements rather 
>> than discussion pieces. I'm working through them.
>>
>>> On Wed, Jul 17, 2019 at 10:03:21 -0400, Collin Walling wrote:
>>>> When baselining CPU models and the user appends the --features argument 
>>>> to the command, s390x will only report back features that supersede the 
>>>> base model definition.
>>>>
>>>> *NOTE* if the --features flag is intended to expand /ALL/ features
>>>> available to a CPU model (such as the huge list of features reported
>>>> by a full CPU model expansion), please let me know and I can resolve 
>>>> this.
>>>
>>> I'm not sure how well this fits s390 world, but the semantics of
>>> VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES flag is to report all CPU
>>> features which are hidden behind the CPU model. That is, all feature
>>> which you'd get when starting QEMU with just the CPU model name and no
>>> additional features. The extra features should not be touched at all.
>>> Specifically, removing them when the flag is not used is wrong.
>>>
>>> To me this looks like the flag should really result in running
>>> query-cpu-model-expansion (but likely the "static" one rather then
>>> "full" expansion) on the baseline CPU model and reporting the enabled
>>> features along with those already included in the baseline feature set.
>>>
>>
>> Actually, query-cpu-model-baseline will return a CPU model along with a
>> feature set. The features returned are the same as those produced from a 
>> static expansion on the model.
>>
>> Correct me if I am wrong here: virsh should report features *only* if the
>> --features flag is present. Otherwise, we only report the model name (which
>> can be accomplished by stripping the result of all reported features).
> 
> No, this is wrong in general (although it maybe correct for s390, I
> don't know).

David here: Not correct for s390x

> 
> Let me explain with some hypothetical CPU models.
> 
> Model1 (when used as -cpu Model1) will enable features f1, f2, f3.
> Model2 will enable f1, f2, f3, f4, f5.
> Model3 will enable f1, f2, f3, f4, f6.
> 
> Running baseline command for [Model1, Model2, Model3] should return
> Model1 with no additional features as [f1, f2, f3] are in only ones
> common to all three models and they are all enabled by Model1.
> 
> However, baseline of [Model2, Model3] should return Model1 + f4. The
> common set of features is [f1, f2, f3, f4], but Model1 would enable [f1,
> f2, f3] only, which means we need to add f4 explicitly.
> 
> With --features, even the features enabled implicitly by a specific CPU
> model should be returned. That is, the first result would be Model1 +
> [f1, f2, f3] and the second result would be Model1 + [f1, f2, f3, f4].
> 
> In other words, stripping all features if no --features option is used
> is wrong. Only features implicitly enabled by the CPU model should be
> stripped.
> 
> Specifically, baseline without --features on the following CPU
> definition
> 
>     <cpu mode='custom' match='exact'>
>       <model>Model1</model>
>       <feature name='f4' policy='require'/>
>     </cpu>
> 
> should be the same definition (i.e., Model1 + f4).

s390x will fallback to base models when baselining/expanding ("minimum
feature set we expect to be around for a certain cpu generation", which
is independent of the QEMU version and will never change). But if you're
already using base models, it works as you would expect it.

E.g.

Model1 is [f1, f2, f3]
Model1-base is [f1, f2]

Model2 is [f1, f2, f3, f4]
Model2-base is [f1, f2, f3]

Baselining Model1 with Model2 would result in
Model1-base + f3

For this, QMP "query-cpu-model-baseline" can be used.

Note: When expending cpu models (e.g. host), one would already always
get base models. So there, it would work completely as you expect it.

Now, to achieve the "--features" part, simply throw the result of
"query-cpu-model-baseline" into "query-cpu-model-expansion" with
"CpuModelExpansionType" == "full"

> 
> The question is whether you can enabled extra features on s390 or you
> can only use plain CPU models with no additional features. If additional
> features are allowed, I'd guess baseline without features should return
> the result of the QMP command minus the features a static expansion of
> the CPU model itself would return (this is assuming f4 will be included
> in the result we get from QEMU for Model1 + f4). When --features is
> used, the result from QEMU should be returned.

We do have plenty of extra cpu features on s390x. *Plenty* :)

> 
> I hope it's clear now. However, I'm not sure how CPU models work on
> s390 and I'd be happy if you could explain it to me. Especially, whether
> something similar to what I described can happen there.
> 
> Jirka
> 


-- 

Thanks,

David / dhildenb