From: Andy Lutomirski <luto@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>
Cc: Tony Luck <tony.luck@intel.com>,
Fenghua Yu <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>,
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: Wed, 29 Sep 2021 09:59:42 -0700 [thread overview]
Message-ID: <75e95acc-6730-ddcf-d722-66e575076256@kernel.org> (raw)
In-Reply-To: <87r1d78t2e.ffs@tglx>
On 9/29/21 05:28, Thomas Gleixner wrote:
> On Wed, Sep 29 2021 at 11:54, Peter Zijlstra wrote:
>> On Fri, Sep 24, 2021 at 04:03:53PM -0700, Andy Lutomirski wrote:
>>> 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.
>>
>> What do we need 1 for? Any PASID change can be achieved using 2 no?
>>
>> Basically, call task_work_add() on all relevant tasks [1], then IPI
>> spray the current running of those and presto.
>>
>> [1] it is nigh on impossible to find all tasks sharing an mm in any sane
>> way due to CLONE_MM && !CLONE_THREAD.
>
> Why would we want any of that at all?
>
> Process starts, no PASID assigned.
>
> bind to device -> PASID is allocated and assigned to the mm
>
> some task of the process issues ENQCMD -> #GP -> write PASID MSR
>
> After that the PASID is saved and restored as part of the XSTATE and
> there is no extra overhead in context switch or return to user space.
>
> All tasks of the process which did never use ENQCMD don't care and their
> PASID xstate is in init state.
>
> There is absolutely no point in enforcing that all tasks of the process
> have the PASID activated immediately when it is assigned. If they need
> it they get it via the #GP fixup and everything just works.
>
> Looking at that patch again, none of this muck in fpu__pasid_write() is
> required at all. The whole exception fixup is:
>
> if (!user_mode(regs))
> return false;
>
> if (!current->mm->pasid)
> return false;
>
> if (current->pasid_activated)
> return false;
<-- preemption or BH here: kaboom.
>
> wrmsrl(MSR_IA32_PASID, current->mm->pasid);
This needs the actual sane fpstate writing helper -- see other email.
next prev parent reply other threads:[~2021-09-29 16:59 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 [this message]
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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