From: "Andy Lutomirski" <luto@kernel.org>
To: "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>,
"Tony Luck" <tony.luck@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>
Cc: 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 16:09:18 -0700 [thread overview]
Message-ID: <d5f00e34-f9fc-4421-8571-48c45237141c@www.fastmail.com> (raw)
In-Reply-To: <20210920192349.2602141-6-fenghua.yu@intel.com>
On Mon, Sep 20, 2021, at 12:23 PM, Fenghua Yu wrote:
> PASIDs are fundamentally hardware resources in a shared address space.
> There is a limited number of them to use ENQCMD on shared workqueue.
> They must be shared and managed. They can not, for instance, be
> statically allocated to processes.
>
> Free PASID eagerly by sending IPIs in unbind was disabled due to locking
> and other issues in commit 9bfecd058339 ("x86/cpufeatures: Force disable
> X86_FEATURE_ENQCMD and remove update_pasid()").
>
> Lazy PASID free is implemented in order to re-enable the ENQCMD feature.
> PASIDs are currently reference counted and are centered around device
> usage. To support lazy PASID free, reference counts are tracked in the
> following scenarios:
>
> 1. The PASID's reference count is initialized as 1 when the PASID is first
> allocated in bind. This is already implemented.
> 2. A reference is taken when a device is bound to the mm and dropped
> when the device is unbound from the mm. This reference tracks device
> usage of the PASID. This is already implemented.
> 3. A reference is taken when a task's IA32_PASID MSR is initialized in
> #GP fix up and dropped when the task exits. This reference tracks
> the task usage of the PASID. It is implemented here.
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 :)
--Andy
next prev parent reply other threads:[~2021-09-23 23:09 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 [this message]
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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