Re: [PATCH v5 02/21] arm64: mm: make vabits_actual a build time constant if possible

From: Mark Rutland <mark.rutland@arm.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>
Subject: Re: [PATCH v5 02/21] arm64: mm: make vabits_actual a build time constant if possible
Date: Sun, 26 Jun 2022 10:31:32 +0100	[thread overview]
Message-ID: <YrgndBReUXJhK/oY@FVFF77S0Q05N> (raw)
In-Reply-To: <20220624150651.1358849-3-ardb@kernel.org>

On Fri, Jun 24, 2022 at 05:06:32PM +0200, Ard Biesheuvel wrote:
> Currently, we only support 52-bit virtual addressing on 64k pages
> 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.
> 
> While at it, move the assignment out of the asm entry code - it has no
> need to be there.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

I see the patch itself checks VA_BITS rather than PAGE_SIZE, (and the former is
the right thing to for FEAT_LPA2), so FWIW:

Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  arch/arm64/include/asm/memory.h |  4 ++++
>  arch/arm64/kernel/head.S        | 15 +--------------
>  arch/arm64/mm/init.c            | 15 ++++++++++++++-
>  arch/arm64/mm/mmu.c             |  4 +++-
>  4 files changed, 22 insertions(+), 16 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
>  	ldr_l	x0, vabits_actual
>  	cmp	x0, #52
>  	b.ne	2f
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 339ee84e5a61..1faa6760895e 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -265,7 +265,20 @@ early_param("mem", early_mem);
>  
>  void __init arm64_memblock_init(void)
>  {
> -	s64 linear_region_size = PAGE_END - _PAGE_OFFSET(vabits_actual);
> +	s64 linear_region_size;
> +
> +#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
> +
> +	linear_region_size = PAGE_END - _PAGE_OFFSET(vabits_actual);
>  
>  	/*
>  	 * Corner case: 52-bit VA capable systems running KVM in nVHE mode may
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 7148928e3932..a6392656d589 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);
> -- 
> 2.35.1
> 

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel