From: "Andy Lutomirski" <luto@kernel.org>
To: "Tony Luck" <tony.luck@intel.com>,
"Thomas Gleixner" <tglx@linutronix.de>
Cc: "Fenghua Yu" <fenghua.yu@intel.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
"Dave Hansen" <dave.hansen@intel.com>,
"Lu Baolu" <baolu.lu@linux.intel.com>,
"Joerg Roedel" <joro@8bytes.org>,
"Josh Poimboeuf" <jpoimboe@redhat.com>,
"Dave Jiang" <dave.jiang@intel.com>,
"Jacob Jun Pan" <jacob.jun.pan@intel.com>,
"Raj Ashok" <ashok.raj@intel.com>,
"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
iommu@lists.linux-foundation.org,
"the arch/x86 maintainers" <x86@kernel.org>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting
Date: Fri, 24 Sep 2021 16:03:53 -0700 [thread overview]
Message-ID: <a77ee33c-6fa7-468c-8fc0-a0a2ce725e75@www.fastmail.com> (raw)
In-Reply-To: <YU3414QT0J7EN4w9@agluck-desk2.amr.corp.intel.com>
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.
next prev parent reply other threads:[~2021-09-24 23:04 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-20 19:23 [PATCH 0/8] Re-enable ENQCMD and PASID MSR Fenghua Yu
2021-09-20 19:23 ` [PATCH 1/8] iommu/vt-d: Clean up unused PASID updating functions Fenghua Yu
2021-09-29 7:34 ` Lu Baolu
2021-09-30 0:40 ` Fenghua Yu
2021-09-20 19:23 ` [PATCH 2/8] x86/process: Clear PASID state for a newly forked/cloned thread Fenghua Yu
2021-09-20 19:23 ` [PATCH 3/8] sched: Define and initialize a flag to identify valid PASID in the task Fenghua Yu
2021-09-20 19:23 ` [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP Fenghua Yu
2021-09-22 21:07 ` Peter Zijlstra
2021-09-22 21:11 ` Peter Zijlstra
2021-09-22 21:26 ` Luck, Tony
2021-09-23 7:03 ` Peter Zijlstra
2021-09-22 21:33 ` Dave Hansen
2021-09-23 7:05 ` Peter Zijlstra
2021-09-22 21:36 ` Fenghua Yu
2021-09-22 23:39 ` Fenghua Yu
2021-09-23 17:14 ` Luck, Tony
2021-09-24 13:37 ` Peter Zijlstra
2021-09-24 15:39 ` Luck, Tony
2021-09-29 9:00 ` Peter Zijlstra
2021-09-23 11:31 ` Thomas Gleixner
2021-09-23 23:17 ` Andy Lutomirski
2021-09-24 2:56 ` Fenghua Yu
2021-09-24 5:12 ` Andy Lutomirski
2021-09-27 21:02 ` Luck, Tony
2021-09-27 23:51 ` Dave Hansen
2021-09-28 18:50 ` Luck, Tony
2021-09-28 19:19 ` Dave Hansen
2021-09-28 20:28 ` Luck, Tony
2021-09-28 20:55 ` Dave Hansen
2021-09-28 23:10 ` Luck, Tony
2021-09-28 23:50 ` Fenghua Yu
2021-09-29 0:08 ` Luck, Tony
2021-09-29 0:26 ` Yu, Fenghua
2021-09-29 1:06 ` Luck, Tony
2021-09-29 1:16 ` Fenghua Yu
2021-09-29 2:11 ` Luck, Tony
2021-09-29 1:56 ` Yu, Fenghua
2021-09-29 2:15 ` Luck, Tony
2021-09-29 16:58 ` Andy Lutomirski
2021-09-29 17:07 ` Luck, Tony
2021-09-29 17:48 ` Andy Lutomirski
2021-09-20 19:23 ` [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting Fenghua Yu
2021-09-23 5:43 ` Lu Baolu
2021-09-30 0:44 ` Fenghua Yu
2021-09-23 14:36 ` Thomas Gleixner
2021-09-23 16:40 ` Luck, Tony
2021-09-23 17:48 ` Thomas Gleixner
2021-09-24 13:18 ` Thomas Gleixner
2021-09-24 16:12 ` Luck, Tony
2021-09-24 23:03 ` Andy Lutomirski [this message]
2021-09-24 23:11 ` Luck, Tony
2021-09-29 9:54 ` Peter Zijlstra
2021-09-29 12:28 ` Thomas Gleixner
2021-09-29 16:51 ` Luck, Tony
2021-09-29 17:07 ` Fenghua Yu
2021-09-29 16:59 ` Andy Lutomirski
2021-09-29 17:15 ` Thomas Gleixner
2021-09-29 17:41 ` Luck, Tony
2021-09-29 17:46 ` Andy Lutomirski
2021-09-29 18:07 ` Fenghua Yu
2021-09-29 18:31 ` Luck, Tony
2021-09-29 20:07 ` Thomas Gleixner
2021-09-24 16:12 ` Fenghua Yu
2021-09-25 23:13 ` Thomas Gleixner
2021-09-28 16:36 ` Fenghua Yu
2021-09-23 23:09 ` Andy Lutomirski
2021-09-23 23:22 ` Luck, Tony
2021-09-24 5:17 ` Andy Lutomirski
2021-09-20 19:23 ` [PATCH 6/8] x86/cpufeatures: Re-enable ENQCMD Fenghua Yu
2021-09-20 19:23 ` [PATCH 7/8] tools/objtool: Check for use of the ENQCMD instruction in the kernel Fenghua Yu
2021-09-22 21:03 ` Peter Zijlstra
2021-09-22 23:44 ` Fenghua Yu
2021-09-23 7:17 ` Peter Zijlstra
2021-09-23 15:26 ` Fenghua Yu
2021-09-24 0:55 ` Josh Poimboeuf
2021-09-24 0:57 ` Fenghua Yu
2021-09-20 19:23 ` [PATCH 8/8] docs: x86: Change documentation for SVA (Shared Virtual Addressing) Fenghua Yu
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