Re: [PATCH] arm64: mm: fix runtime fallback to 48-bt VA when 52-bit VA is enabled

[Ard Biesheuvel]

(+ James)

On Wed, 10 Mar 2021 at 13:41, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Wed, 10 Mar 2021 at 12:18, Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > On Tue, Mar 09, 2021 at 07:32:16PM -0500, Mark Salter wrote:
> > > I ran into an early boot soft lockup on a Qualcomm Amberwing using a v5.11
> > > kernel configured for 52-bit VA. This turned into a panic with a v5.12-rc2
> > > kernel.
> > >
> > > The problem is that when we fall back to 48-bit VA, idmap_t0sz is not
> > > updated. Later, the kvm hypervisor uses idmap_t0sz to set its tcr_el2 and
> > > hangs (v5.11). After commit 1401bef703a4 ("arm64: mm: Always update TCR_EL1
> > > from __cpu_set_tcr_t0sz()"), the kernel panics when trying to use the idmap
> > > to call idmap_cpu_replace_ttbr1().
> > >
> > > Oddly, other systems (thunderX2 and Ampere eMag) which don't support 52-bit
> > > VA seem to handle the setting of an unsupported t0sz without any apparent
> > > problems. Indeed, if one reads back the tcr written with t0sz==12, the
> > > value read has t0sz==16. Not so with Amberwing.
> >
> > Nice, you have one of those elusive platforms!
> >
> > > Fixes: 90ec95cda91a ("arm64: mm: Introduce VA_BITS_MIN")
> > > Signed-off-by: Mark Salter <msalter@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/kernel/head.S | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > > index 66b0e0b66e31..2bcbbb26292e 100644
> > > --- a/arch/arm64/kernel/head.S
> > > +++ b/arch/arm64/kernel/head.S
> > > @@ -291,6 +291,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> > >        */
> > >       adrp    x0, idmap_pg_dir
> > >       adrp    x3, __idmap_text_start          // __pa(__idmap_text_start)
> > > +     mov     x4, TCR_T0SZ(VA_BITS)
> > >
> > >  #ifdef CONFIG_ARM64_VA_BITS_52
> > >       mrs_s   x6, SYS_ID_AA64MMFR2_EL1
> > > @@ -299,6 +300,13 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> > >       cbnz    x6, 1f
> > >  #endif
> > >       mov     x5, #VA_BITS_MIN
> > > +#ifdef CONFIG_ARM64_VA_BITS_52
> > > +     mov     x4, TCR_T0SZ(VA_BITS_MIN)
> > > +     adr_l   x6, idmap_t0sz
> > > +     str     x4, [x6]
> > > +     dmb     sy
> > > +     dc      ivac, x6                // Invalidate potentially stale cache line
> > > +#endif
> > >  1:
> > >       adr_l   x6, vabits_actual
> > >       str     x5, [x6]
> > > @@ -319,7 +327,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> > >        */
> > >       adrp    x5, __idmap_text_end
> > >       clz     x5, x5
> > > -     cmp     x5, TCR_T0SZ(VA_BITS)   // default T0SZ small enough?
> > > +     cmp     x5, x4                  // default T0SZ small enough?
> > >       b.ge    1f                      // .. then skip VA range extension
> >
> > Could we instead have the default value be 48-bit, and then avoid having
> > to update the variable in both cases? e.g. something along the lines of
> > the entirely untested diff below?
> >
>
> There is one other occurrence that needs to be updated. I will have a
> stab at fixing this along these lines, and there are a couple of other
> things that need cleaning up.
>

Actually, it seems like this breakage may have been introduced by

commit 1401bef703a48cf79c674215f702a1435362beae
Author: James Morse <james.morse@xxxxxxx>
Date:   Mon Jan 25 14:19:12 2021 -0500

    arm64: mm: Always update TCR_EL1 from __cpu_set_tcr_t0sz()

as before that patch, we would never actually load idmap_t0sz into TCR
on 52-bit VA kernels, AFAICT.

It seems to me that the sanest approach is to default to a 48-bit
idmap on 52-bit VA capable kernels (as Will suggests below). This
means that the notion of 'extended ID map' now also includes cases
where the ID map is smaller than the default VA_BITS range. However,
the number of levels is going to be the same, so it does not affect
the ID map page table population code in head.S.

Also, __cpu_uses_extended_idmap() no longer has any users anyway, so
we can remove it rather than bikeshed over a better name for it.


>
>
> > --->8
> >
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index 66b0e0b66e31..fb795123896f 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -319,7 +319,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> >          */
> >         adrp    x5, __idmap_text_end
> >         clz     x5, x5
> > -       cmp     x5, TCR_T0SZ(VA_BITS)   // default T0SZ small enough?
> > +       cmp     x5, TCR_T0SZ(VA_BITS_MIN)       // default T0SZ small enough?
> >         b.ge    1f                      // .. then skip VA range extension
> >
> >         adr_l   x6, idmap_t0sz
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 3802cfbdd20d..4c5603c41870 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -40,7 +40,7 @@
> >  #define NO_BLOCK_MAPPINGS      BIT(0)
> >  #define NO_CONT_MAPPINGS       BIT(1)
> >
> > -u64 idmap_t0sz = TCR_T0SZ(VA_BITS);
> > +u64 idmap_t0sz = TCR_T0SZ(VA_BITS_MIN);
> >  u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
> >
> >  u64 __section(".mmuoff.data.write") vabits_actual;
> >
> >
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