Re: [PATCH 2/3] x86/split_lock: Bits in IA32_CORE_CAPABILITIES are not architectural

[Luck, Tony]

On Fri, Apr 17, 2020 at 12:06:47AM +0200, Thomas Gleixner wrote:
> Tony Luck <tony.luck@xxxxxxxxx> writes:
> > Features enumerated by IA32_CORE_CAPABILITIES are model specific and
> > implementation details may vary in different cpu models. Thus it is only
> > safe to trust features after checking the CPU model.
> 
> What's the point of the IA32_CORE_CAPABILITIES check if we need a model
> match to figure out whether IA32_CORE_CAPABILITIES bit 5 is valid to
> enumerate split lock detection?
> 
> IOW, are we going to see CPUs which end up in the match list and have
> bit 5 cleared in IA32_CORE_CAPABILITIES?

There may be low-end SKUs of a model that don't have all the features of
the high-end SKUs. So yes, you may find that a specific SKU of a model
on the list for a feature doesn't have the feature.

A model specific feature may also have implementation differences
on different models.  E.g. if Intel were to produce a model that
did split lock "right" (with thread-scoped control). That would
still use the same bit in IA32_CORE_CAPABILITIES, but the OS would
need model specific knowledge to know that this split lock detect
worked differently from another model that has split lock detect.

-Tony