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Message-ID: <e932c4c4-c0a5-76f0-20ee-1d0155d5b79e@arm.com>
Date:   Mon, 17 Apr 2023 16:48:02 +0100
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Subject: Re: [PATCH v3 32/60] arm64: head: allocate more pages for the kernel
 mapping
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To:     Ard Biesheuvel <ardb@kernel.org>, linux-kernel@vger.kernel.org
Cc:     linux-arm-kernel@lists.infradead.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Kees Cook <keescook@chromium.org>
References: <20230307140522.2311461-1-ardb@kernel.org>
 <20230307140522.2311461-33-ardb@kernel.org>
From:   Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <20230307140522.2311461-33-ardb@kernel.org>
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Precedence: bulk

On 07/03/2023 14:04, Ard Biesheuvel wrote:
> In preparation for switching to an early kernel mapping routine that
> maps each segment according to its precise boundaries, and with the
> correct attributes, let's allocate some extra pages for page tables for
> the 4k page size configuration. This is necessary because the start and
> end of each segment may not be aligned to the block size, and so we'll
> need an extra page table at each segment boundary.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/include/asm/kernel-pgtable.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
> index 4d13c73171e1e360..50b5c145358a5d8e 100644
> --- a/arch/arm64/include/asm/kernel-pgtable.h
> +++ b/arch/arm64/include/asm/kernel-pgtable.h
> @@ -80,7 +80,7 @@
>  			+ EARLY_PGDS((vstart), (vend), add) 	/* each PGDIR needs a next level page table */	\
>  			+ EARLY_PUDS((vstart), (vend), add)	/* each PUD needs a next level page table */	\
>  			+ EARLY_PMDS((vstart), (vend), add))	/* each PMD needs a next level page table */
> -#define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR))
> +#define INIT_DIR_SIZE (PAGE_SIZE * (EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR) + EARLY_SEGMENT_EXTRA_PAGES))
>  
>  /* the initial ID map may need two extra pages if it needs to be extended */
>  #if VA_BITS < 48
> @@ -101,6 +101,15 @@
>  #define SWAPPER_TABLE_SHIFT	PMD_SHIFT
>  #endif
>  
> +/* The number of segments in the kernel image (text, rodata, inittext, initdata, data+bss) */
> +#define KERNEL_SEGMENT_COUNT	5
> +
> +#if SWAPPER_BLOCK_SIZE > SEGMENT_ALIGN
> +#define EARLY_SEGMENT_EXTRA_PAGES (KERNEL_SEGMENT_COUNT + 1)

I'm guessing the block size for 4K pages is PMD, so you need these extra pages
to define PTEs for the case where the section start/end addresses are not on
exact 2MB boundaries? But in that case, isn't it possible that you would need 2
extra PTE tables per segment - one for the start and one for the end?

> +#else
> +#define EARLY_SEGMENT_EXTRA_PAGES 0
> +#endif
> +
>  /*
>   * Initial memory map attributes.
>   */