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

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Date:   Fri, 24 Sep 2021 16:03:53 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Tony Luck" <tony.luck@...el.com>,
        "Thomas Gleixner" <tglx@...utronix.de>
Cc:     "Fenghua Yu" <fenghua.yu@...el.com>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.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>,
        "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 9:12 AM, Luck, Tony wrote:
> On Fri, Sep 24, 2021 at 03:18:12PM +0200, Thomas Gleixner wrote:
>> 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.
>
> It would be possible to avoid a per-task reference to the PASID by
> taking an extra reference when mm->pasid is first allocated using
> the CONFIG_PASID_TASK_REFS you proposed yesterday to define a function
> to take the extra reference, and another to drop it when the mm is
> finally freed ... with stubs to do nothing on architectures that
> don't create per-task PASID context.
>
> This solution works, but is functionally different from Fenghua's
> proposal for this case:
>
> 	Process clones a task
> 	task binds a device
> 	task accesses device using PASID
> 	task unbinds device
> 	task exits (but process lives on)
>
> Fenghua will free the PASID because the reference count goes
> back to zero. The "take an extra reference and keep until the
> mm is freed" option would needlessly hold onto the PASID.
>
> This seems like an odd usage case ... even if it does exist, a process
> that does this may spawn another task that does the same thing.
>
> If you think it is sufficiently simpler to use the "extra reference"
> option (invoking the "perfect is the enemy of good enough" rule) then we
> can change.

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.

And there is still no per-task refcounting.

After all, the not so perfect attempt at perfect in this patch set won't actually work if a thread pool is involved.