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

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Message-ID: <87k0j6dsdn.ffs@tglx>
Date:   Fri, 24 Sep 2021 15:18:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting

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.

Thanks,

        tglx