linux-kernel - Re: [PATCH v2] arm64/kernel: Fix range on invalidating dcache for boot page tables

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Message-ID: <20200428112157.GB15519@C02TD0UTHF1T.local>
Date:   Tue, 28 Apr 2020 12:21:57 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Gavin Shan <gshan@...hat.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        steve.capper@....com, catalin.marinas@....com, broonie@...nel.org,
        will@...nel.org, shan.gavin@...il.com
Subject: Re: [PATCH v2] arm64/kernel: Fix range on invalidating dcache for
 boot page tables

On Tue, Apr 28, 2020 at 09:57:00AM +1000, Gavin Shan wrote:
> Prior to commit 8eb7e28d4c642c31 ("arm64/mm: move runtime pgds to
> rodata"), idmap_pgd_dir, tramp_pg_dir, reserved_ttbr0, swapper_pg_dir,
> and init_pg_dir were contiguous at the end of the kernel image. The
> maintenance at the end of __create_page_tables assumed these were
> contiguous, and affected everything from the start of idmap_pg_dir
> to the end of init_pg_dir.
> 
> That commit moved all but init_pg_dir into the .rodata section, with
> other data placed between idmap_pg_dir and init_pg_dir, but did not
> update the maintenance. Hence the maintenance is performed on much
> more data than necessary (but as the bootloader previously made this
> clean to the PoC there is no functional problem).
> 
> As we only alter idmap_pg_dir, and init_pg_dir, we only need to perform
> maintenance for these. As the other dirs are in .rodata, the bootloader
> will have initialised them as expected and cleaned them to the PoC. The
> kernel will initialize them as necessary after enabling the MMU.
> 
> This patch reworks the maintenance to only cover the idmap_pg_dir and
> init_pg_dir to avoid this unnecessary work.
> 
> Signed-off-by: Gavin Shan <gshan@...hat.com>

Reviewed-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
> v2: Include the suggested commit log    (Mark Rutland)
>     Improved comments and code          (Mark Rutland)
> ---
>  arch/arm64/include/asm/pgtable.h |  1 +
>  arch/arm64/kernel/head.S         | 12 +++++++++---
>  arch/arm64/kernel/vmlinux.lds.S  |  1 +
>  3 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 8c20e2bd6287..5caff09c6a3a 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -457,6 +457,7 @@ extern pgd_t init_pg_dir[PTRS_PER_PGD];
>  extern pgd_t init_pg_end[];
>  extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
>  extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
> +extern pgd_t idmap_pg_end[];
>  extern pgd_t tramp_pg_dir[PTRS_PER_PGD];
>  
>  extern void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd);
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 57a91032b4c2..32f5ecbec0ea 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -394,13 +394,19 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
>  
>  	/*
>  	 * Since the page tables have been populated with non-cacheable
> -	 * accesses (MMU disabled), invalidate the idmap and swapper page
> -	 * tables again to remove any speculatively loaded cache lines.
> +	 * accesses (MMU disabled), invalidate those tables again to
> +	 * remove any speculatively loaded cache lines.
>  	 */
> +	dmb	sy
> +
>  	adrp	x0, idmap_pg_dir
> +	adrp	x1, idmap_pg_end
> +	sub	x1, x1, x0
> +	bl	__inval_dcache_area
> +
> +	adrp	x0, init_pg_dir
>  	adrp	x1, init_pg_end
>  	sub	x1, x1, x0
> -	dmb	sy
>  	bl	__inval_dcache_area
>  
>  	ret	x28
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 497f9675071d..94402aaf5f5c 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -139,6 +139,7 @@ SECTIONS
>  
>  	idmap_pg_dir = .;
>  	. += IDMAP_DIR_SIZE;
> +	idmap_pg_end = .;
>  
>  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>  	tramp_pg_dir = .;
> -- 
> 2.23.0
>