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

["Andy Lutomirski" <luto@kernel.org>]

On Mon, Sep 20, 2021, at 12:23 PM, Fenghua Yu wrote:
> PASIDs are fundamentally hardware resources in a shared address space.
> There is a limited number of them to use ENQCMD on shared workqueue.
> They must be shared and managed. They can not, for instance, be
> statically allocated to processes.
>
> Free PASID eagerly by sending IPIs in unbind was disabled due to locking
> and other issues in commit 9bfecd058339 ("x86/cpufeatures: Force disable
> X86_FEATURE_ENQCMD and remove update_pasid()").
>
> Lazy PASID free is implemented in order to re-enable the ENQCMD feature.
> PASIDs are currently reference counted and are centered around device
> usage. To support lazy PASID free, reference counts are tracked in the
> following scenarios:
>
> 1. The PASID's reference count is initialized as 1 when the PASID is first
>    allocated in bind. This is already implemented.
> 2. A reference is taken when a device is bound to the mm and dropped
>    when the device is unbound from the mm. This reference tracks device
>    usage of the PASID. This is already implemented.
> 3. A reference is taken when a task's IA32_PASID MSR is initialized in
>    #GP fix up and dropped when the task exits. This reference tracks
>    the task usage of the PASID. It is implemented here.

I think this is unnecessarily complicated because it's buying in to the existing ISA misconception that PASID has anything to do with a task.  A PASID belongs to an mm, full stop.  Now the ISA is nasty and we have tasks that have *noticed* that their mm has a PASID and tasks that have not noticed this fact, but that should be irrelevant to essentially everything except the fault handler.

So just refcount the thing the obvious way: take a reference when you stick the PASID in the mm_struct and drop the reference in __mmdrop().  Problem solved.  You could probably drop it more aggressively in __mmput(), and the comment explaining why is left as an exercise to the reader -- if a kernel thread starts doing ENQCMD, we have worse things to worry about :)

--Andy