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

Palmer Dabbelt
2020-09-19T04:41:31Z

On Mon, 14 Sep 2020 20:58:13 PDT (-0700), greentime.hu@sifive.com wrote:
> Palmer Dabbelt <palmer@dabbelt.com> 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@brainfault.org wrote:
>> > On Tue, Aug 4, 2020 at 8:32 AM Greentime Hu <greentime.hu@sifive.com> 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@sifive.com>
>> >> Signed-off-by: Syven Wang <syven.wang@sifive.com>
>> >> Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
>> >> ---
>> >> 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 barriers used for page table modifications") is out of date, as I can't find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!