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
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