linux-kernel - Re: [PATCH 2/8] arm64: kernel: use literal for relocated address of __secondary

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Date:	Mon, 18 Apr 2016 16:57:33 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	will.deacon@....com, james.morse@....com, catalin.marinas@....com
Subject: Re: [PATCH 2/8] arm64: kernel: use literal for relocated address of
 __secondary_switched

On Mon, Apr 18, 2016 at 05:09:42PM +0200, Ard Biesheuvel wrote:
> We can simply use a relocated 64-bit literal to store the address of
> __secondary_switched(), and the relocation code will ensure that it
> holds the correct value at secondary entry time, as long as we make sure
> that the literal is not dereferenced until after we have enabled the MMU.
> 
> So jump via a small __secondary_switch() function covered by the ID map
> that performs the literal load and branch-to-register.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

Neat!

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

Mark.

> ---
>  arch/arm64/kernel/head.S | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index ac27d8d937b2..f13276d4ca91 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -468,9 +468,7 @@ __mmap_switched:
>  	str	x15, [x11, x23]
>  	b	0b
>  
> -2:	adr_l	x8, kimage_vaddr		// make relocated kimage_vaddr
> -	dc	cvac, x8			// value visible to secondaries
> -	dsb	sy				// with MMU off
> +2:
>  #endif
>  
>  	adr_l	sp, initial_sp, x4
> @@ -699,12 +697,9 @@ secondary_startup:
>  	adrp	x26, swapper_pg_dir
>  	bl	__cpu_setup			// initialise processor
>  
> -	ldr	x8, kimage_vaddr
> -	ldr	w9, 0f
> -	sub	x27, x8, w9, sxtw		// address to jump to after enabling the MMU
> +	adr_l	x27, __secondary_switch		// address to jump to after enabling the MMU
>  	b	__enable_mmu
>  ENDPROC(secondary_startup)
> -0:	.long	(_text - TEXT_OFFSET) - __secondary_switched
>  
>  __secondary_switched:
>  	adr_l	x5, vectors
> @@ -806,3 +801,8 @@ __no_granule_support:
>  	wfi
>  	b 1b
>  ENDPROC(__no_granule_support)
> +
> +__secondary_switch:
> +	ldr	x8, =__secondary_switched
> +	br	x8
> +ENDPROC(__secondary_switch)
> -- 
> 2.5.0
>