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Date:   Tue, 15 Dec 2020 21:33:15 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Anton Vorontsov <anton.vorontsov@...aro.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] powerpc/64s: Trim offlined CPUs from mm_cpumasks

Nicholas Piggin <npiggin@...il.com> writes:
> Excerpts from Michael Ellerman's message of December 14, 2020 8:43 pm:
>> Nicholas Piggin <npiggin@...il.com> writes:
>>> Excerpts from Geert Uytterhoeven's message of December 10, 2020 7:06 pm:
>>>> Hi Nicholas,
>>>> 
>>>> On Fri, Nov 20, 2020 at 4:01 AM Nicholas Piggin <npiggin@...il.com> wrote:
>>>>>
>>>>> When offlining a CPU, powerpc/64s does not flush TLBs, rather it just
>>>>> leaves the CPU set in mm_cpumasks, so it continues to receive TLBIEs
>>>>> to manage its TLBs.
>>>>>
>>>>> However the exit_flush_lazy_tlbs() function expects that after
>>>>> returning, all CPUs (except self) have flushed TLBs for that mm, in
>>>>> which case TLBIEL can be used for this flush. This breaks for offline
>>>>> CPUs because they don't get the IPI to flush their TLB. This can lead
>>>>> to stale translations.
>>>>>
>>>>> Fix this by clearing the CPU from mm_cpumasks, then flushing all TLBs
>>>>> before going offline.
>>>>>
>>>>> These offlined CPU bits stuck in the cpumask also prevents the cpumask
>>>>> from being trimmed back to local mode, which means continual broadcast
>>>>> IPIs or TLBIEs are needed for TLB flushing. This patch prevents that
>>>>> situation too.
>>>>>
>>>>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>>>> 
>>>> Thanks for your patch!
>>>> 
>>>>> --- a/arch/powerpc/platforms/powermac/smp.c
>>>>> +++ b/arch/powerpc/platforms/powermac/smp.c
>>>>> @@ -911,6 +911,8 @@ static int smp_core99_cpu_disable(void)
>>>>>
>>>>>         mpic_cpu_set_priority(0xf);
>>>>>
>>>>> +       cleanup_cpu_mmu_context();
>>>>> +
>>>> 
>>>> I guess this change broke pmac32_defconfig+SMP in v5.10-rc7?
>>>> 
>>>> arch/powerpc/platforms/powermac/smp.c: error: implicit
>>>> declaration of function 'cleanup_cpu_mmu_context'
>>>> [-Werror=implicit-function-declaration]:  => 914:2
>>>> 
>>>> http://kisskb.ellerman.id.au/kisskb/buildresult/14423174/
>>>
>>> Hey, yeah it does thanks for catching it. This patch fixes it for me
>>>
>>> ---
>>> From a9b5ec92ffac975e81c6d7db6ff2b1486b2723f7 Mon Sep 17 00:00:00 2001
>>> From: Nicholas Piggin <npiggin@...il.com>
>>> Date: Mon, 14 Dec 2020 13:52:39 +1000
>>> Subject: [PATCH] powerpc/32s: Fix cleanup_cpu_mmu_context() compile bug
>>>
>>> 32s has no tlbiel_all() defined, so just disable the cleanup with a
>>> comment.
>> 
>> Or what about just:
>
> That works, I kind of wanted it in there explicit that we don't
> clean up on 32s. I don't mind if you prefer this though.

OK. I'll merge my version because I can do that without needing to merge
master in order to get the broken commit into my tree.

cheers

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