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

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Message-ID: <1ef4c737-0926-424c-9698-794c23370b74@arm.com>
Date: Mon, 12 Feb 2024 13:05:51 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 12/02/2024 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.

https://lore.kernel.org/linux-arm-kernel/123a58b0-2ea6-4da3-9719-98ca55c8095e@arm.com/

> 
> Maybe wort spelling out in the patch description
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
> 

Thanks!