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

[Palmer Dabbelt]

On Mon, 14 Sep 2020 20:58:13 PDT (-0700), greentime.hu@xxxxxxxxxx wrote:
> Palmer Dabbelt <palmer@xxxxxxxxxxx> 於 2020年8月5日 週三 上午10:03寫道：
> > 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 barriers used for page table modifications") is out of date, as I can't find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?
>
> Hi Palmer,
>
> Some architectures add cache writeback in set_pte(), just like nds32, csky.
>
> https://github.com/torvalds/linux/blob/master/arch/nds32/include/asm/pgtable.h#L213
> https://github.com/torvalds/linux/blob/master/arch/csky/include/asm/pgtable.h#L104
>
> Would you like to pick this patch or should I send another patch to
> implement it in set_pte()?

Sorry, I forgot about this.  I'm picking up this patch on fixes, as it clearly
fixes a bug.  We might also have uncovered another bug, but there's no sense in
letting this one hang around until we figure that out.

I've cleaned up the comment text a bit, as Anup pointed out some issuses.

Thanks!