[RFC PATCH 3/3] arm64: Handle AArch32 tasks running on non AArch32 cpu

[Peter Zijlstra]

On Fri, Oct 09, 2020 at 10:13:12AM +0200, Morten Rasmussen wrote:
> On Fri, Oct 09, 2020 at 09:29:43AM +0200, Peter Zijlstra wrote:

> > Fundamentally, you're not supposed to change the userspace provided
> > affinity mask. If we want to do something like this, we'll have to teach
> > the scheduler about this second mask such that it can compute an
> > effective mask as the intersection between the 'feature' and user mask.
> 
> I agree that we shouldn't mess wit the user-space mask directly. Would it
> be unthinkable to go down the route of maintaining a new mask which is
> the intersection of the feature mask (controlled and updated by arch
> code) and the user-space mask?
> 
> It shouldn't add overhead in the scheduler as it would use the
> intersection mask instead of the user-space mask, the main complexity
> would be around making sure the intersection mask is updated correctly
> (cpusets, hotplug, ...).

IFF we _need_ to go there, then yes that was the plan, compose the
intersection when either the (arch) feature(set) mask or the userspace
mask changes.