From: Fenghua Yu <fenghua.yu@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Luck, Tony" <tony.luck@intel.com>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Peter Zijlstra <peterz@infradead.org>,
Andy Lutomirski <luto@kernel.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>,
Ashok Raj <ashok.raj@intel.com>,
Ravi V Shankar <ravi.v.shankar@intel.com>,
iommu@lists.linux-foundation.org, x86 <x86@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting
Date: Fri, 24 Sep 2021 16:12:43 +0000 [thread overview]
Message-ID: <YU34+1J4v0cn9ZRs@otcwcpicx3.sc.intel.com> (raw)
In-Reply-To: <87k0j6dsdn.ffs@tglx>
Hi, Thomas,
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.
There are up to 1M PASIDs because PASID is 20-bit. I think there are a few ways
to allocate and free PASID:
1. Statically allocate a PASID once a mm is created and free it in mm
exit. No PASID allocation/free during the mm's lifetime. Then
up to 1M processes can be created due to 1M PASIDs limitation.
We don't want this method because the 1M processes limitation.
2. A PASID is allocated to the mm in open(dev)->bind(dev, mm). There
are three ways to free it:
(a) Actively free it in close(fd)->unbind(dev, mm) by sending
IPIs to tell all tasks using the PASID to clear the IA32_PASID
MSR. This has locking issues similar to the actively loading
IA32_PASID MSR which was force disabled in upstream. So won't work.
(b) Passively free the PASID in destroy_context(mm) in mm exit. Once
the PASID is allocated, it stays with the process for the lifetime. It's
better than #1 because the PASID is allocated only on demand.
(c) Passively free the PASID in deactive_mm(mm) or unbind() whenever there
is no usage as implemented in this series. Tracking the PASID usage
per task provides a chance to free the PASID on task exit. The
PASID has a better chance to be freed earlier than mm exit in #(b).
This series uses #2 and #(c) to allocate and free the PASID for a better
chance to ease the 1M PASIDs limitation pressure. For example, a thread
doing open(dev)->ENQCMD->close(fd)->exit(2) will not occupy a PASID while
its sibling threads are still running.
Thanks.
-Fenghua
next prev parent reply other threads:[~2021-09-24 16:12 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
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 [this message]
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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