Re: [PATCH 1/4] riscv: tlb: fix __p*d_free_tlb()

[Alexandre Ghiti]

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@xxxxxxxxxx>
> ---
>   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();

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 */
>