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
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