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

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Date:   Wed, 29 Sep 2021 11:54:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Tony Luck <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting

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.