Re: [PATCH] Insert SFENCE.VMA in function set_pte_at for RISCV

[Anup Patel <anup@brainfault.org>]

+Alex

On Tue, Mar 16, 2021 at 9:20 AM Jiuyang Liu <liu@jiuyang.me> wrote:
>
> This patch inserts SFENCE.VMA after modifying PTE based on RISC-V
> specification.
>
> arch/riscv/include/asm/pgtable.h:
> 1. implement pte_user, pte_global and pte_leaf to check correspond
> attribute of a pte_t.

Adding pte_user(), pte_global(), and pte_leaf() is fine.

>
> 2. insert SFENCE.VMA in set_pte_at based on RISC-V Volume 2, Privileged
> Spec v. 20190608 page 66 and 67:
> If software modifies a non-leaf PTE, it should execute SFENCE.VMA with
> rs1=x0. If any PTE along the traversal path had its G bit set, rs2 must
> be x0; otherwise, rs2 should be set to the ASID for which the
> translation is being modified.
> If software modifies a leaf PTE, it should execute SFENCE.VMA with rs1
> set to a virtual address within the page. If any PTE along the traversal
> path had its G bit set, rs2 must be x0; otherwise, rs2 should be set to
> the ASID for which the translation is being modified.
>
> arch/riscv/include/asm/tlbflush.h:
> 1. implement get_current_asid to get current program asid.
> 2. implement local_flush_tlb_asid to flush tlb with asid.

As per my understanding, we don't need to explicitly invalidate local TLB
in set_pte() or set_pet_at() because generic Linux page table management
(<linux>/mm/*) will call the appropriate flush_tlb_xyz() function after page
table updates. Also, just local TLB flush is generally not sufficient because
a lot of page tables will be used accross on multiple HARTs.

>
> Signed-off-by: Jiuyang Liu <liu@jiuyang.me>
> ---
>  arch/riscv/include/asm/pgtable.h  | 27 +++++++++++++++++++++++++++
>  arch/riscv/include/asm/tlbflush.h | 12 ++++++++++++
>  2 files changed, 39 insertions(+)
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index ebf817c1bdf4..5a47c60372c1 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -222,6 +222,16 @@ static inline int pte_write(pte_t pte)
>         return pte_val(pte) & _PAGE_WRITE;
>  }
>
> +static inline int pte_user(pte_t pte)
> +{
> +       return pte_val(pte) & _PAGE_USER;
> +}
> +
> +static inline int pte_global(pte_t pte)
> +{
> +       return pte_val(pte) & _PAGE_GLOBAL;
> +}
> +
>  static inline int pte_exec(pte_t pte)
>  {
>         return pte_val(pte) & _PAGE_EXEC;
> @@ -248,6 +258,11 @@ static inline int pte_special(pte_t pte)
>         return pte_val(pte) & _PAGE_SPECIAL;
>  }
>
> +static inline int pte_leaf(pte_t pte)
> +{
> +       return pte_val(pte) & (_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC);
> +}
> +
>  /* static inline pte_t pte_rdprotect(pte_t pte) */
>
>  static inline pte_t pte_wrprotect(pte_t pte)
> @@ -358,6 +373,18 @@ static inline void set_pte_at(struct mm_struct *mm,
>                 flush_icache_pte(pteval);
>
>         set_pte(ptep, pteval);
> +
> +       if (pte_present(pteval)) {
> +               if (pte_leaf(pteval)) {
> +                       local_flush_tlb_page(addr);
> +               } else {
> +                       if (pte_global(pteval))
> +                               local_flush_tlb_all();
> +                       else
> +                               local_flush_tlb_asid();
> +
> +               }
> +       }
>  }
>
>  static inline void pte_clear(struct mm_struct *mm,
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index 394cfbccdcd9..1f9b62b3670b 100644
> --- a/arch/riscv/include/asm/tlbflush.h
> +++ b/arch/riscv/include/asm/tlbflush.h
> @@ -21,6 +21,18 @@ static inline void local_flush_tlb_page(unsigned long addr)
>  {
>         __asm__ __volatile__ ("sfence.vma %0" : : "r" (addr) : "memory");
>  }
> +
> +static inline unsigned long get_current_asid(void)
> +{
> +       return (csr_read(CSR_SATP) >> SATP_ASID_SHIFT) & SATP_ASID_MASK;
> +}
> +
> +static inline void local_flush_tlb_asid(void)
> +{
> +       unsigned long asid = get_current_asid();
> +       __asm__ __volatile__ ("sfence.vma x0, %0" : : "r" (asid) : "memory");
> +}
> +
>  #else /* CONFIG_MMU */
>  #define local_flush_tlb_all()                  do { } while (0)
>  #define local_flush_tlb_page(addr)             do { } while (0)
> --
> 2.30.2
>
>
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Regards,
Anup