# Re: Supporting core-specific instruction sets (e.g. big.LITTLE) with restartable sequences

- From: Florian Weimer \<hidden\>
- Date: 2018-11-02 19:18:40 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2018/11/5422486
- Thread: https://ratatoskr.run/linux-api/2018/11/5422486/t.md

```text
* Mathieu Desnoyers:

> Basically, the use-cases targeted are those where some cores on the
> system support a larger instruction set than others. So for instance,
> some cores could use a faster atomic add instruction than others,
> which should rely on a slower fallback. This is also the same story
> for reading the performance monitoring unit counters from user-space:
> it depends on the feature-set supported by the CPU on which the
> instruction is issued. Same applies to cores having different
> cache-line sizes.

The kernel needs to present a consistent view to userspace, the common
denominator.  I don't think there is any other way.

The situation is not new at all, by the way.  It also arises with VM and
process migration.  In glibc, we do not re-run CPU feature selection
upon resume (and how could we? function pointers would have to change),
and we have no plans to implement anything differently.

Thanks,
Florian
```