Re: [PATCH] riscv: Add sfence.vma after page table changed

[Palmer Dabbelt]

On Mon, 03 Aug 2020 20:29:32 PDT (-0700), anup@xxxxxxxxxxxxxx wrote:
> On Tue, Aug 4, 2020 at 8:32 AM Greentime Hu <greentime.hu@xxxxxxxxxx> wrote:
> > This patch addes local_flush_tlb_page(addr) to use sfence.vma after the
>
> s/addes/adds
>
> > page table changed. That address will be used immediately in
> > memset(nextp, 0, PAGE_SIZE) to cause this issue so we should add the
> > sfence.vma before we use it.
>
> Alternate version of this commit description can be:
>
> Invalidate local TLB after both set_pet() and clear_pte() because the
> address can be used immediately after page table change.
>
> > Fixes: f2c17aabc917 ("RISC-V: Implement compile-time fixed mappings")
> >
> > Reported-by: Syven Wang <syven.wang@xxxxxxxxxx>
> > Signed-off-by: Syven Wang <syven.wang@xxxxxxxxxx>
> > Signed-off-by: Greentime Hu <greentime.hu@xxxxxxxxxx>
> > ---
> >  arch/riscv/mm/init.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index f4adb3684f3d..29b0f7108054 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -202,12 +202,11 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
> >
> >         ptep = &fixmap_pte[pte_index(addr)];
> >
> > -       if (pgprot_val(prot)) {
> > +       if (pgprot_val(prot))
> >                 set_pte(ptep, pfn_pte(phys >> PAGE_SHIFT, prot));
> > -       } else {
> > +       else
> >                 pte_clear(&init_mm, addr, ptep);
> > -               local_flush_tlb_page(addr);
> > -       }
> > +       local_flush_tlb_page(addr);
> >  }

arm64 appears to be upgrading all set_pte()s on valid kernel mappings to
include the fence.  It looks like the message from 7f0b1bf04511 ("arm64: Fix
barriers used for page table modifications") is out of date, as I can't find
create_mapping() any more.  If that was some generic kernel thing then we
should probably upgrade ours as well, but if it was arch/arm64/ code then this
approach seems fine as __set_fixmap() isn't on the hot path -- I guess this is
fine either way, but there may be other issues that the arm64 approach fixes.

Do you guys happen to remember what was going on here?

> >  static pte_t *__init get_pte_virt(phys_addr_t pa)
> > --
> > 2.28.0
>
> Otherwise looks good to me.
>
> Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>
>
> Regards,
> Anup