Date:   Wed, 10 Mar 2021 13:41:16 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Mark Salter <msalter@...hat.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] arm64: mm: fix runtime fallback to 48-bt VA when 52-bit
 VA is enabled

On Wed, 10 Mar 2021 at 12:18, Will Deacon <will@...nel.org> 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@...hat.com>
> > ---
> >  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.



> --->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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