Re: [External] [PATCH v6 08/13] riscv: Avoid TLB flush loops when affected by SiFive CIP-1200

[yunhui cui]

Hi Samuel,

On Wed, Mar 27, 2024 at 12:51 PM Samuel Holland
<samuel.holland@xxxxxxxxxx> wrote:
>
> Implementations affected by SiFive errata CIP-1200 have a bug which
> forces the kernel to always use the global variant of the sfence.vma
> instruction. When affected by this errata, do not attempt to flush a
> range of addresses; each iteration of the loop would actually flush the
> whole TLB instead. Instead, minimize the overall number of sfence.vma
> instructions.
>
> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> ---
>
> Changes in v6:
>  - Clarify the commit message for patch 8 based on ML discussion
>
> Changes in v4:
>  - Only set tlb_flush_all_threshold when CONFIG_MMU=y.
>
> Changes in v3:
>  - New patch for v3
>
>  arch/riscv/errata/sifive/errata.c | 5 +++++
>  arch/riscv/include/asm/tlbflush.h | 2 ++
>  arch/riscv/mm/tlbflush.c          | 2 +-
>  3 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/errata/sifive/errata.c b/arch/riscv/errata/sifive/errata.c
> index 3d9a32d791f7..716cfedad3a2 100644
> --- a/arch/riscv/errata/sifive/errata.c
> +++ b/arch/riscv/errata/sifive/errata.c
> @@ -42,6 +42,11 @@ static bool errata_cip_1200_check_func(unsigned long  arch_id, unsigned long imp
>                 return false;
>         if ((impid & 0xffffff) > 0x200630 || impid == 0x1200626)
>                 return false;
> +
> +#ifdef CONFIG_MMU
> +       tlb_flush_all_threshold = 0;
> +#endif
> +
>         return true;
>  }
>
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index 463b615d7728..8e329721375b 100644
> --- a/arch/riscv/include/asm/tlbflush.h
> +++ b/arch/riscv/include/asm/tlbflush.h
> @@ -66,6 +66,8 @@ void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
>                                unsigned long uaddr);
>  void arch_flush_tlb_batched_pending(struct mm_struct *mm);
>  void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
> +
> +extern unsigned long tlb_flush_all_threshold;
>  #else /* CONFIG_MMU */
>  #define local_flush_tlb_all()                  do { } while (0)
>  #endif /* CONFIG_MMU */
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index ad7bdcfcc219..18af7b5053af 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -11,7 +11,7 @@
>   * Flush entire TLB if number of entries to be flushed is greater
>   * than the threshold below.
>   */
> -static unsigned long tlb_flush_all_threshold __read_mostly = 64;
> +unsigned long tlb_flush_all_threshold __read_mostly = 64;
>
>  static void local_flush_tlb_range_threshold_asid(unsigned long start,
>                                                  unsigned long size,
> --
> 2.43.1
>

Reviewed-by: Yunhui Cui <cuiyunhui@xxxxxxxxxxxxx>

Thanks,
Yunhui