linux-kernel - Re: [PATCH v1] arm64: mm: Batch dsb and isb when populating pgtables

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Message-ID: <a6da59d1-28b5-4cc0-aae9-fca1286be577@arm.com>
Date: Thu, 28 Mar 2024 08:45:14 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Mark Rutland <mark.rutland@....com>, David Hildenbrand <david@...hat.com>,
 Donald Dutile <ddutile@...hat.com>, Eric Chanudet <echanude@...hat.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] arm64: mm: Batch dsb and isb when populating pgtables

On 28/03/2024 07:23, Ard Biesheuvel wrote:
> On Wed, 27 Mar 2024 at 21:07, Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> After removing uneccessary TLBIs, the next bottleneck when creating the
>> page tables for the linear map is DSB and ISB, which were previously
>> issued per-pte in __set_pte(). Since we are writing multiple ptes in a
>> given pte table, we can elide these barriers and insert them once we
>> have finished writing to the table.
>>
> 
> Nice!
> 
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>  arch/arm64/include/asm/pgtable.h |  7 ++++++-
>>  arch/arm64/mm/mmu.c              | 13 ++++++++++++-
>>  2 files changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index bd5d02f3f0a3..81e427b23b3f 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -271,9 +271,14 @@ static inline pte_t pte_mkdevmap(pte_t pte)
>>         return set_pte_bit(pte, __pgprot(PTE_DEVMAP | PTE_SPECIAL));
>>  }
>>
>> -static inline void __set_pte(pte_t *ptep, pte_t pte)
>> +static inline void ___set_pte(pte_t *ptep, pte_t pte)
> 
> IMHO, we should either use WRITE_ONCE() directly in the caller, or
> find a better name.

How about __set_pte_nosync() ?

> 
>>  {
>>         WRITE_ONCE(*ptep, pte);
>> +}
>> +
>> +static inline void __set_pte(pte_t *ptep, pte_t pte)
>> +{
>> +       ___set_pte(ptep, pte);
>>
>>         /*
>>          * Only if the new pte is valid and kernel, otherwise TLB maintenance
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 1b2a2a2d09b7..c6d5a76732d4 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -301,7 +301,11 @@ static pte_t *init_pte(pte_t *ptep, unsigned long addr, unsigned long end,
>>         do {
>>                 pte_t old_pte = __ptep_get(ptep);
>>
>> -               __set_pte(ptep, pfn_pte(__phys_to_pfn(phys), prot));
>> +               /*
>> +                * Required barriers to make this visible to the table walker
>> +                * are deferred to the end of alloc_init_cont_pte().
>> +                */
>> +               ___set_pte(ptep, pfn_pte(__phys_to_pfn(phys), prot));
>>
>>                 /*
>>                  * After the PTE entry has been populated once, we
>> @@ -358,6 +362,13 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>>         } while (addr = next, addr != end);
>>
>>         ops->unmap(TYPE_PTE);
>> +
>> +       /*
>> +        * Ensure all previous pgtable writes are visible to the table walker.
>> +        * See init_pte().
>> +        */
>> +       dsb(ishst);
>> +       isb();
>>  }
>>
>>  static pmd_t *init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
>> --
>> 2.25.1
>>