linux-kernel - Re: [PATCH 1/4] riscv: tlb: fix __p*d_free

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Date: Wed, 7 Feb 2024 08:28:01 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Jisheng Zhang <jszhang@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Will Deacon <will@...nel.org>,
 "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Nick Piggin <npiggin@...il.com>,
 Peter Zijlstra <peterz@...radead.org>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/4] riscv: tlb: fix __p*d_free_tlb()

Hi Jisheng,

On 31/12/2023 07:21, Alexandre Ghiti wrote:
> Hi Jisheng,
>
> On 19/12/2023 18:50, Jisheng Zhang wrote:
>> If non-leaf PTEs I.E pmd, pud or p4d is modified, a sfence.vma is
>> a must for safe, imagine if an implementation caches the non-leaf
>> translation in TLB, although I didn't meet this HW so far, but it's
>> possible in theory.
>>
>> Signed-off-by: Jisheng Zhang<jszhang@...nel.org>
>> ---
>>   arch/riscv/include/asm/pgalloc.h | 20 +++++++++++++++++---
>>   1 file changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/pgalloc.h 
>> b/arch/riscv/include/asm/pgalloc.h
>> index d169a4f41a2e..a12fb83fa1f5 100644
>> --- a/arch/riscv/include/asm/pgalloc.h
>> +++ b/arch/riscv/include/asm/pgalloc.h
>> @@ -95,7 +95,13 @@ static inline void pud_free(struct mm_struct *mm, 
>> pud_t *pud)
>>           __pud_free(mm, pud);
>>   }
>>   -#define __pud_free_tlb(tlb, pud, addr) pud_free((tlb)->mm, pud)
>> +#define __pud_free_tlb(tlb, pud, addr)                    \
>> +do {                                    \
>> +    if (pgtable_l4_enabled) {                    \
>> +        pagetable_pud_dtor(virt_to_ptdesc(pud));        \
>> +        tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(pud)); \
>
>
> The specification indeed states that an sfence.vma must be emitted 
> after a page directory modification. Your change is not enough though 
> since eventually tlb_flush() is called and in this function we should 
> add:
>
> if (tlb->freed_tables)
>     tlb_flush_mm();


I sent a patch for that here 
https://lore.kernel.org/linux-riscv/20240128120405.25876-1-alexghiti@rivosinc.com/

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks,

Alex


>
> otherwise we are not guaranteed that a "global" sfence.vma is called.
>
> Would you be able to benchmark this change and see the performance 
> impact?
>
> Thanks,
>
> Alex
>
>
>> +    }                                \
>> +} while (0)
>>     #define p4d_alloc_one p4d_alloc_one
>>   static inline p4d_t *p4d_alloc_one(struct mm_struct *mm, unsigned 
>> long addr)
>> @@ -124,7 +130,11 @@ static inline void p4d_free(struct mm_struct 
>> *mm, p4d_t *p4d)
>>           __p4d_free(mm, p4d);
>>   }
>>   -#define __p4d_free_tlb(tlb, p4d, addr) p4d_free((tlb)->mm, p4d)
>> +#define __p4d_free_tlb(tlb, p4d, addr)                    \
>> +do {                                    \
>> +    if (pgtable_l5_enabled)                        \
>> +        tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(p4d)); \
>> +} while (0)
>>   #endif /* __PAGETABLE_PMD_FOLDED */
>>     static inline void sync_kernel_mappings(pgd_t *pgd)
>> @@ -149,7 +159,11 @@ static inline pgd_t *pgd_alloc(struct mm_struct 
>> *mm)
>>     #ifndef __PAGETABLE_PMD_FOLDED
>>   -#define __pmd_free_tlb(tlb, pmd, addr) pmd_free((tlb)->mm, pmd)
>> +#define __pmd_free_tlb(tlb, pmd, addr)                \
>> +do {                                \
>> +    pagetable_pmd_dtor(virt_to_ptdesc(pmd));        \
>> +    tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(pmd));    \
>> +} while (0)
>>     #endif /* __PAGETABLE_PMD_FOLDED */
>