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

[Fenghua Yu]

Hi, Thomas,

On Fri, Sep 24, 2021 at 03:18:12PM +0200, Thomas Gleixner wrote:
> On Thu, Sep 23 2021 at 19:48, Thomas Gleixner wrote:
> > On Thu, Sep 23 2021 at 09:40, Tony Luck wrote:
> >
> > fpu_write_task_pasid() can just grab the pasid from current->mm->pasid
> > and be done with it.
> >
> > The task exit code can just call iommu_pasid_put_task_ref() from the
> > generic code and not from within x86.
> 
> But OTOH why do you need a per task reference count on the PASID at all?
> 
> The PASID is fundamentaly tied to the mm and the mm can't go away before
> the threads have gone away unless this magically changed after I checked
> that ~20 years ago.

There are up to 1M PASIDs because PASID is 20-bit. I think there are a few ways
to allocate and free PASID:

1. Statically allocate a PASID once a mm is created and free it in mm
   exit. No PASID allocation/free during the mm's lifetime. Then
   up to 1M processes can be created due to 1M PASIDs limitation.
   We don't want this method because the 1M processes limitation.

2. A PASID is allocated to the mm in open(dev)->bind(dev, mm). There
   are three ways to free it:
   (a) Actively free it in close(fd)->unbind(dev, mm) by sending
       IPIs to tell all tasks using the PASID to clear the IA32_PASID
       MSR. This has locking issues similar to the actively loading
       IA32_PASID MSR which was force disabled in upstream. So won't work.
   (b) Passively free the PASID in destroy_context(mm) in mm exit. Once
       the PASID is allocated, it stays with the process for the lifetime. It's
       better than #1 because the PASID is allocated only on demand.
   (c) Passively free the PASID in deactive_mm(mm) or unbind() whenever there
       is no usage as implemented in this series. Tracking the PASID usage
       per task provides a chance to free the PASID on task exit. The
       PASID has a better chance to be freed earlier than mm exit in #(b).

This series uses #2 and #(c) to allocate and free the PASID for a better
chance to ease the 1M PASIDs limitation pressure. For example, a thread
doing open(dev)->ENQCMD->close(fd)->exit(2) will not occupy a PASID while
its sibling threads are still running.

Thanks.

-Fenghua