Re: [PATCH] riscv: mm: synchronize MMU after page table update

[Guo Ren]

On Tue, Mar 24, 2020 at 1:50 PM Nick Hu <nickhu@xxxxxxxxxxxxx> wrote:
>
> Similar to commit bf587caae305 ("riscv: mm: synchronize MMU after pte change")
>
> For those riscv implementations whose TLB cannot synchronize with dcache,
> an SFENCE.VMA is necessary after page table update.
> This patch fixed two functions:
>
> 1. pgd_alloc
> During fork, a parent process prepares pgd for its child and updates satp
> later, but they may not run on the same core. Adding a remote SFENCE.VMA to
> invalidate TLB in other cores is needed. Thus use flush_tlb_all() instead
> of local_flush_tlb_all() here.
> Similar approaches can be found in arm and csky.
>
> 2. __set_fixmap
> Add a SFENCE.VMA after fixmap pte update.
> Similar approaches can be found in arm and sh.
>
> Signed-off-by: Nick Hu <nickhu@xxxxxxxxxxxxx>
> Signed-off-by: Nylon Chen <nylon7@xxxxxxxxxxxxx>
> Cc: Alan Kao <alankao@xxxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/pgalloc.h | 1 +
>  arch/riscv/mm/init.c             | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> index 3f601ee8233f..071468fa14b7 100644
> --- a/arch/riscv/include/asm/pgalloc.h
> +++ b/arch/riscv/include/asm/pgalloc.h
> @@ -51,6 +51,7 @@ static inline pgd_t *pgd_alloc(struct mm_struct *mm)
>                 memcpy(pgd + USER_PTRS_PER_PGD,
>                         init_mm.pgd + USER_PTRS_PER_PGD,
>                         (PTRS_PER_PGD - USER_PTRS_PER_PGD) * sizeof(pgd_t));
> +               flush_tlb_all();
Are you sure to put a tlb flush operation here ? I think it should be
a dcache flush ops (but there is no solution for riscv ISA :P)

Ref to csky's implementation, just some old cores need sync dcache
with tlb by a simple dcache flush range ops and tlb ops should be put
in another place due to Linux infrastructure.

static inline pgd_t *pgd_alloc(struct mm_struct *mm)
{
        pgd_t *ret;
        pgd_t *init;

        ret = (pgd_t *) __get_free_pages(GFP_KERNEL, PGD_ORDER);
        if (ret) {
                init = pgd_offset(&init_mm, 0UL);
                pgd_init((unsigned long *)ret);
                memcpy(ret + USER_PTRS_PER_PGD, init + USER_PTRS_PER_PGD,
                        (PTRS_PER_PGD - USER_PTRS_PER_PGD) * sizeof(pgd_t));
                /* prevent out of order excute */
                smp_mb();
#ifdef CONFIG_CPU_NEED_TLBSYNC
                dcache_wb_range((unsigned int)ret,
                                (unsigned int)(ret + PTRS_PER_PGD));
#endif
        }

        return ret;
}


>         }
>         return pgd;
>  }
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index fab855963c73..a7f329503ed0 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -203,8 +203,8 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
>                 set_pte(ptep, pfn_pte(phys >> PAGE_SHIFT, prot));
>         } else {
>                 pte_clear(&init_mm, addr, ptep);
> -               local_flush_tlb_page(addr);
>         }
> +       local_flush_tlb_page(addr);
>  }
>
>  static pte_t *__init get_pte_virt(phys_addr_t pa)
> --
> 2.17.0
>
>


-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/