From: "Andy Lutomirski" <luto@kernel.org>
To: "Tony Luck" <tony.luck@intel.com>
Cc: "Fenghua Yu" <fenghua.yu@intel.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"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: Thu, 23 Sep 2021 22:17:25 -0700 [thread overview]
Message-ID: <bdb8246a-e157-4ceb-a410-90eebd4043e7@www.fastmail.com> (raw)
In-Reply-To: <YU0MO/WJO4rkcl0c@agluck-desk2.amr.corp.intel.com>
On Thu, Sep 23, 2021, at 4:22 PM, Luck, Tony wrote:
> On Thu, Sep 23, 2021 at 04:09:18PM -0700, Andy Lutomirski wrote:
>> On Mon, Sep 20, 2021, at 12:23 PM, Fenghua Yu wrote:
>
>> I think this is unnecessarily complicated because it's buying in to the
>> existing ISA misconception that PASID has anything to do with a task.
>> A PASID belongs to an mm, full stop. Now the ISA is nasty and we have
>> tasks that have *noticed* that their mm has a PASID and tasks that have
>> not noticed this fact, but that should be irrelevant to essentially
>> everything except the fault handler.
>>
>> So just refcount the thing the obvious way: take a reference when you
>> stick the PASID in the mm_struct and drop the reference in __mmdrop().
>> Problem solved. You could probably drop it more aggressively in
>> __mmput(), and the comment explaining why is left as an exercise to the
>> reader -- if a kernel thread starts doing ENQCMD, we have worse things
>> to worry about :)
>
> That doesn't match well with the non-x86 usage of PASIDs. The code there
> bumps the reference count on each device bind, and decrements on each
> device unbind.
Can you elaborate on how that works? Is there an architecture where there is a bona fide per task PASID?
>
> If we don't keep a reference count for each task that has IA32_PASID
> set up we could have this sequence
>
> 1) Process binds to a PASID capable device
Okay, so the mm has that PASID set up and a reference is taken.
> 2) Task uses ENQCMD, so PASID MSR is set up.
Yep.
> 3) Process unbinds the device, reference count on PASID
> goes to zero. PASID is freed. PASID is reallocated to
> another task.
It had better not. We had an entire phone call in which we agreed that the entire lazy-MSR-setup approach only makes any sense if everyone pinky swears that an mm will *never* change its PASID once it has a PASID.
> 4) Task from step #2 uses ENQCMD to submit a descriptor
> and device now processes virtual addresses based on mappings
> in the new task.
>
> Now you might say that at step 3 we need to hunt down all the
> tasks that have PASID enabled and disabled ... but that's the
> same painful code that we avoided when we said that we would
> not make Linux hand out a PASID to all existing tasks in a
> process on the first bind operation.
>
Exactly. Which means that the mm ought to pin that PASID for as long as it exists. What am I missing?
Sure, one can invent a situation in which you start two threads, and one of those threads binds a device, does ENQCMD, unbinds the device, and exits. Then the other thread *in the same mm* binds another device and gets a new PASID. And it all works. But I really don't think this special case is worth optimizing for.
> -Tony
next prev parent reply other threads:[~2021-09-24 5:17 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
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 [this message]
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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