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Date: Tue, 14 Jun 2022 10:34:47 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>,
linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
Will Deacon <will@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Kees Cook <keescook@...omium.org>,
Catalin Marinas <catalin.marinas@....com>,
Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v4 02/26] arm64: mm: make vabits_actual a build time
constant if possible
On Tue, 14 Jun 2022 at 10:25, Anshuman Khandual
<anshuman.khandual@....com> wrote:
>
>
> On 6/13/22 20:15, Ard Biesheuvel wrote:
> > Currently, we only support 52-bit virtual addressing on 64k pages
>
> But going forward, will support on 4K/16K pages as well via FEAT_LPA2.
>
> > configurations, and in all other cases, vabits_actual is guaranteed to
> > equal VA_BITS (== VA_BITS_MIN). So get rid of the variable entirely in
> > that case.
>
> The change here does not really get rid of vabit_actual in those cases
> either, it just makes it a build time constant AFAICS.
>
Indeed, and so it ceases to be a variable.
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -174,7 +174,11 @@
> #include <linux/types.h>
> #include <asm/bug.h>
>
> +#if VA_BITS > 48
> extern u64 vabits_actual;
> +#else
> +#define vabits_actual ((u64)VA_BITS)
> +#endif
>
> >
> > While at it, move the assignment out of the asm entry code - it has no
> > need to be there.
>
> This also changes when vabits_actual gets evaluated ? Then how would it
> know, that CPU needs to be stuck in kernel (CPU_STUCK_REASON_52_BIT_VA)
> in case all secondary CPUs do not support large VA feature ? Looking at
> the sequence...
>
> secondary_entry
> OR
> secondary_holding_pen
> secondary_startup
> __cpu_secondary_check52bitva
>
> primary_entry
> __create_page_tables <--- original position
> __primary_switch
> start_kernel
> setup_arch
> paging_init <--- new position
>
> It might still be possible for the secondary cpu start up sequence to
> validate LVA support across the platform, but still why even send
> vabits_actual evaluation down the line until paging_init(). Ideally
> should not it be evaluated as early as possible during boot. Hence,
> wondering - what is the real benefit here ?
>
Why should it be evaluated as early as possible? The whole point is
deferring it so we don't have to do it from asm code.
But I suppose doing it as early as possible from C code (i.e., in
setup_arch() before arm64_memblock_init() or even before
early_fixmap_init()) might be better.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> > arch/arm64/include/asm/memory.h | 4 ++++
> > arch/arm64/kernel/head.S | 15 +--------------
> > arch/arm64/mm/mmu.c | 15 ++++++++++++++-
> > 3 files changed, 19 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > index 0af70d9abede..c751cd9b94f8 100644
> > --- a/arch/arm64/include/asm/memory.h
> > +++ b/arch/arm64/include/asm/memory.h
> > @@ -174,7 +174,11 @@
> > #include <linux/types.h>
> > #include <asm/bug.h>
> >
> > +#if VA_BITS > 48
> > extern u64 vabits_actual;
> > +#else
> > +#define vabits_actual ((u64)VA_BITS)
> > +#endif
> >
> > extern s64 memstart_addr;
> > /* PHYS_OFFSET - the physical address of the start of memory. */
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index 1cdecce552bb..dc07858eb673 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -293,19 +293,6 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> > adrp x0, idmap_pg_dir
> > adrp x3, __idmap_text_start // __pa(__idmap_text_start)
> >
> > -#ifdef CONFIG_ARM64_VA_BITS_52
> > - mrs_s x6, SYS_ID_AA64MMFR2_EL1
> > - and x6, x6, #(0xf << ID_AA64MMFR2_LVA_SHIFT)
> > - mov x5, #52
> > - cbnz x6, 1f
> > -#endif
> > - mov x5, #VA_BITS_MIN
> > -1:
> > - adr_l x6, vabits_actual
> > - str x5, [x6]
> > - dmb sy
> > - dc ivac, x6 // Invalidate potentially stale cache line
> > -
> > /*
> > * VA_BITS may be too small to allow for an ID mapping to be created
> > * that covers system RAM if that is located sufficiently high in the
> > @@ -713,7 +700,7 @@ SYM_FUNC_START(__enable_mmu)
> > SYM_FUNC_END(__enable_mmu)
> >
> > SYM_FUNC_START(__cpu_secondary_check52bitva)
> > -#ifdef CONFIG_ARM64_VA_BITS_52
> > +#if VA_BITS > 48
>
> Just curious - why this is any better ? Although both (VA_BITS > 48)
> and CONFIG_ARM64_VA_BITS_52 are build time constants.
>
VA_BITS > 48 is a bit more readable, and more likely to remain accurate.
> > ldr_l x0, vabits_actual
> > cmp x0, #52
> > b.ne 2f
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 7148928e3932..17b339c1a326 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -46,8 +46,10 @@
> > u64 idmap_t0sz = TCR_T0SZ(VA_BITS_MIN);
> > u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
> >
> > -u64 __section(".mmuoff.data.write") vabits_actual;
> > +#if VA_BITS > 48
> > +u64 vabits_actual __ro_after_init = VA_BITS_MIN;
> > EXPORT_SYMBOL(vabits_actual);
> > +#endif
> >
> > u64 kimage_vaddr __ro_after_init = (u64)&_text;
> > EXPORT_SYMBOL(kimage_vaddr);
> > @@ -772,6 +774,17 @@ void __init paging_init(void)
> > {
> > pgd_t *pgdp = pgd_set_fixmap(__pa_symbol(swapper_pg_dir));
> >
> > +#if VA_BITS > 48
> > + if (cpuid_feature_extract_unsigned_field(
> > + read_sysreg_s(SYS_ID_AA64MMFR2_EL1),
> > + ID_AA64MMFR2_LVA_SHIFT))
> > + vabits_actual = VA_BITS;
> > +
> > + /* make the variable visible to secondaries with the MMU off */
> > + dcache_clean_inval_poc((u64)&vabits_actual,
> > + (u64)&vabits_actual + sizeof(vabits_actual));
> > +#endif
> > +
> > map_kernel(pgdp);
> > map_mem(pgdp);
> >
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