linux-kernel - Re: [PATCH v5 18/25] arm64/mm: Split __flush_tlb

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Message-ID: <608feac1-8cd6-48c2-87ab-688fb9c0bda4@redhat.com>
Date: Mon, 12 Feb 2024 14:15:02 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>,
 James Morse <james.morse@....com>, Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Mark Rutland <mark.rutland@....com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, John Hubbard
 <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
 Barry Song <21cnbao@...il.com>, Alistair Popple <apopple@...dia.com>,
 Yang Shi <shy828301@...il.com>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>
Cc: linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 18/25] arm64/mm: Split __flush_tlb_range() to elide
 trailing DSB

On 12.02.24 14:05, Ryan Roberts wrote:
> On 12/02/2024 12:44, David Hildenbrand wrote:
>> On 02.02.24 09:07, Ryan Roberts wrote:
>>> Split __flush_tlb_range() into __flush_tlb_range_nosync() +
>>> __flush_tlb_range(), in the same way as the existing flush_tlb_page()
>>> arrangement. This allows calling __flush_tlb_range_nosync() to elide the
>>> trailing DSB. Forthcoming "contpte" code will take advantage of this
>>> when clearing the young bit from a contiguous range of ptes.
>>>
>>> Tested-by: John Hubbard <jhubbard@...dia.com>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>> ---
>>>    arch/arm64/include/asm/tlbflush.h | 13 +++++++++++--
>>>    1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/tlbflush.h
>>> b/arch/arm64/include/asm/tlbflush.h
>>> index 79e932a1bdf8..50a765917327 100644
>>> --- a/arch/arm64/include/asm/tlbflush.h
>>> +++ b/arch/arm64/include/asm/tlbflush.h
>>> @@ -422,7 +422,7 @@ do {                                    \
>>>    #define __flush_s2_tlb_range_op(op, start, pages, stride, tlb_level) \
>>>        __flush_tlb_range_op(op, start, pages, stride, 0, tlb_level, false,
>>> kvm_lpa2_is_enabled());
>>>    -static inline void __flush_tlb_range(struct vm_area_struct *vma,
>>> +static inline void __flush_tlb_range_nosync(struct vm_area_struct *vma,
>>>                         unsigned long start, unsigned long end,
>>>                         unsigned long stride, bool last_level,
>>>                         int tlb_level)
>>> @@ -456,10 +456,19 @@ static inline void __flush_tlb_range(struct
>>> vm_area_struct *vma,
>>>            __flush_tlb_range_op(vae1is, start, pages, stride, asid,
>>>                         tlb_level, true, lpa2_is_enabled());
>>>    -    dsb(ish);
>>>        mmu_notifier_arch_invalidate_secondary_tlbs(vma->vm_mm, start, end);
>>>    }
>>>    +static inline void __flush_tlb_range(struct vm_area_struct *vma,
>>> +                     unsigned long start, unsigned long end,
>>> +                     unsigned long stride, bool last_level,
>>> +                     int tlb_level)
>>> +{
>>> +    __flush_tlb_range_nosync(vma, start, end, stride,
>>> +                 last_level, tlb_level);
>>> +    dsb(ish);
>>> +}
>>> +
>>>    static inline void flush_tlb_range(struct vm_area_struct *vma,
>>>                       unsigned long start, unsigned long end)
>>>    {
>>
>> You're now calling dsb() after mmu_notifier_arch_invalidate_secondary_tlbs().
>>
>>
>> In flush_tlb_mm(), we have the order
>>
>>      dsb(ish);
>>      mmu_notifier_arch_invalidate_secondary_tlbs()
>>
>> In flush_tlb_page(), we have the effective order:
>>
>>      mmu_notifier_arch_invalidate_secondary_tlbs()
>>      dsb(ish);
>>
>> In flush_tlb_range(), we used to have the order:
>>
>>      dsb(ish);
>>      mmu_notifier_arch_invalidate_secondary_tlbs();
>>
>>
>> So I *suspect* having that DSB before
>> mmu_notifier_arch_invalidate_secondary_tlbs() is fine. Hopefully, nothing in
>> there relies on that placement.
> 
> Will spotted this against v3. My argument was that I was following the existing
> pattern in flush_tlb_page(). Apparently that is not correct and needs changing,
> but the conclusion was to leave my change as is for now, since it is consistent
> and change them at a later date together.

Good, I think you should add a few words to the patch description 
("ordering might be incorrect, but is in-line with __flush_tlb_page()"; 
will be resolved separately).

-- 
Cheers,

David / dhildenb