Re: [Qemu-devel] [PATCH 02/10] s390x/cpumodel: remove CSSKE from base model

[Christian Borntraeger]

On 18.04.19 14:45, David Hildenbrand wrote:
> On 18.04.19 13:31, Christian Borntraeger wrote:
>> conditional sske is deprecated and a distant future machine (will be one
>> where the IBC will not allow to fully go back to z14) will remove this
>> feature. To prepare for this and allow for the z14 and older cpu model
>> to still run on systems without csske, remove csske from the base (and
> 
> will csske feature be a default feature for zNext? Or is it not
> available *at all*.
> 
> In case it is not available, baselining and cpu model comparison have to
> be thought about "ignoring csske".
> 
>> thus the default models for z10..z14). For compat machines we have to
>> add those back.
> 
> Base models are machine-independent. That means, changing base models is
> not supported.

Why is that? for the expansion?

> Once we introduce new models like here, we can set the
> new base models into stone.

the new model is easy (and yes I could only disable CSSKE in the base
model for gen15 but not for gen14. 

The problem is that without some kind of fixup for older base models like
z10-z14 expansion will fall back to z9 on anything that no longer has
csske. 

> 
>>
[....]
>>
> 
> Instead of doing that, can we rather start generating the next
> generation "fresh", listing all base model features it contains instead
> of doing it incrementally? Could end up "nicer"
> 
> 
> In target/s390x/cpu_models.c we have:
> 
> "... For now, base features of a following release are always a subset
> of base features of the previous release. Same is correct for the other
> feature sets."
> 
> This is especially relevant for "s390_find_cpu_def", and goes into the
> direction of baselining, as previously mentioned.
> 
> Luckily, we already have "ignored_base_feat", maybe we can simply add
> csske there and have it working.

Yes, maybe we could ignore csske.