Subject: [PATCH 4/4] arm64: align PHYS_OFFSET to block size

From: [email-masked] (Catalin Marinas)
Date: Wed, 25 Mar 2015 14:59:49 +0000

On Mon, Mar 23, 2015 at 04:36:56PM +0100, Ard Biesheuvel wrote:
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 16134608eecf..fd8434753372 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -49,13 +49,15 @@
>  #ifdef CONFIG_ARM64_64K_PAGES
>  #define BLOCK_SHIFT	PAGE_SHIFT
>  #define BLOCK_SIZE	PAGE_SIZE
> -#define TABLE_SHIFT	PMD_SHIFT
>  #else
>  #define BLOCK_SHIFT	SECTION_SHIFT
>  #define BLOCK_SIZE	SECTION_SIZE
> -#define TABLE_SHIFT	PUD_SHIFT
>  #endif
>  
> +#define TABLE_SHIFT	(BLOCK_SHIFT + PAGE_SHIFT - 3)
> +#define TABLE_SIZE	(1 << TABLE_SHIFT)
> +#define TABLE_MASK	(~(TABLE_SIZE - 1))
> +
>  #define KERNEL_START	_text
>  #define KERNEL_END	_end
>  
> @@ -237,7 +239,10 @@ ENTRY(stext)
>  	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
>  
>  	adrp	x24, __PHYS_OFFSET
> -	mov	x23, #KIMAGE_OFFSET
> +	and	x23, x24, #~TABLE_MASK		// image offset
> +	and	x24, x24, #TABLE_MASK		// PHYS_OFFSET
> +	mov	x0, #KIMAGE_OFFSET
> +	add	x23, x23, x0

I'm still trying to figure out how this works. Does the code imply that
the kernel image can only be loaded within a block size of the
PHYS_OFFSET? If that's the case, it's not too flexible.

My thoughts were to make PHYS_OFFSET permanently 0 (i.e. get rid of it
entirely) or at least compute it at DT parsing time. I'm more inclined
for making it 0 assuming that it doesn't break anything else (vmemmap
virtual range may get slightly bigger but still not significant,
basically max_phys_addr / sizeof(struct page)).

-- 
Catalin