Re: [PATCH] riscv: mm: check the SV39 rule

[Cheng Chao <cs.os.kernel@gmail.com>]

Hi Alexandre,
  Thanks for reviewing this patch.  I don't reply on time due to the
National Day :)


On Mon, Sep 30, 2024 at 3:46 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> Hi Cheng,
>
> On 28/09/2024 17:52, Cheng Chao wrote:
> > SV39 rule: the address of bits[63..39] should be the same as bit[38],
> > it is easy to violate if configure PAGE_OFFSET too small.
> > for instance, PAGE_OFFSET=0xffffffc0_0000_0000,
>
>
> Out of curiosity, why do you try to modify the current memory layout?

It's a long story. I'm working on kernel-5.10, and the default
PAGE_OFFSET 0xffffffe0_0000_0000 can only map the 128G physical
address,
and we need to map more than 128G in the real world.

After I changed PAGE_OFFSET to a smaller value which can map a more
physical address.
the kernel panic(page fault) when  earlycon printk,  the cause is 0xf,
badaddr is 0xffffffbx_xxxx_xxxx.
It took me several days to debug this panic, at the beginning, I
considered MMU doesn't work for earlycon, so I traced the pgd/pmd/pte,
all are correct before page fault.
Occasionally, I found the SV39 rule: the address of bits[63..39]
should be the same as bit[38],  the address 0xffffffbx_xxxx_xxxx
violates the rule.
The root cause is that the kernel doesn't sense the SV39 rule, so when
I adjust the PAGE_OFFSET, the FIXADDR_START will adjust too without
any check.
when accessing the [FIXADDR, FIXADDR + PMD_SIZE] , the kernel will
have a page fault.

>
>
> > the FIXADDR_START will be the 0xfffffffb0_xxxx_xxxx,
> > bit[39] == 0'b1, bit[38] == 0'b0, when access the FIXADDR,
> > which cause the page fault.
> > It's difficult to debug, check it when compile is necessary.
> >
> > Signed-off-by: Cheng Chao <cs.os.kernel@gmail.com>
> > ---
> >   arch/riscv/mm/init.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index bfa2dea95354..a5fb3fc2b2db 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -1387,6 +1387,12 @@ static void __init arch_reserve_crashkernel(void)
> >
> >   void __init paging_init(void)
> >   {
> > +#ifdef CONFIG_64BIT
> > +     BUILD_BUG_ON_MSG((VA_BITS == VA_BITS_SV39) &&
>
>
> VA_BITS is determined at runtime, either by setting "noXlvl" on the
> kernel command line or by probing the HW capabilities, so the above
> can't be determined at build time, or am I missing something?
>
you are right,
in case of CONFIG_XIP_KERNEL=y,  VA_BITS will be VA_BITS_SV39,  this
patch works.
other cases, disable_pgtable_l4 will enable SATP_MODE_39,  we also
check SV39 rule here?
@@ -763,6 +763,11 @@ static void __init disable_pgtable_l4(void)
        pgtable_l4_enabled = false;
        kernel_map.page_offset = PAGE_OFFSET_L3;
        satp_mode = SATP_MODE_39;
+
+       if ((VA_BITS == VA_BITS_SV39) &&
+                       (((FIXADDR_START & BIT(39)) >> 39)
+                        != ((FIXADDR_START & BIT(38)) >> 38)))
+               panic("violate SV39 rule: bits[63..39] should be same
as bit[38]\n");
 }

>
> > +                     (((FIXADDR_START & BIT(39)) >> 39)
> > +                      != ((FIXADDR_START & BIT(38)) >> 38)),
> > +                     "violate SV39 rule: bits[63..39] should be same as bit[38]");
> > +#endif
> >       setup_bootmem();
> >       setup_vm_final();
> >
>
>
> Thanks,
>
> Alex
>

Thanks,
Cheng Chao

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