Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting

[Peter Zijlstra]

On Fri, Sep 24, 2021 at 04:03:53PM -0700, Andy Lutomirski wrote:
> I think the perfect and the good are a bit confused here. If we go for
> "good", then we have an mm owning a PASID for its entire lifetime.  If
> we want "perfect", then we should actually do it right: teach the
> kernel to update an entire mm's PASID setting all at once.  This isn't
> *that* hard -- it involves two things:
> 
> 1. The context switch code needs to resync PASID.  Unfortunately, this
> adds some overhead to every context switch, although a static_branch
> could minimize it for non-PASID users.

> 2. A change to an mm's PASID needs to sent an IPI, but that IPI can't
> touch FPU state.  So instead the IPI should use task_work_add() to
> make sure PASID gets resynced.

What do we need 1 for? Any PASID change can be achieved using 2 no?

Basically, call task_work_add() on all relevant tasks [1], then IPI
spray the current running of those and presto.

[1] it is nigh on impossible to find all tasks sharing an mm in any sane
way due to CLONE_MM && !CLONE_THREAD.