Messages in this thread | | | Date | Wed, 29 Sep 2021 11:54:09 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting |
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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.
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