linux-kernel - Re: [PATCH] arm64: head: remove unnecessary instructions from __relocate

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Message-ID: <ZeWThcOa-cW2Ax5a@FVFF77S0Q05N>
Date: Mon, 4 Mar 2024 09:25:38 +0000
From: Mark Rutland <mark.rutland@....com>
To: Puranjay Mohan <puranjay12@...il.com>
Cc: catalin.marinas@....com, will@...nel.org, anshuman.khandual@....com,
	ardb@...nel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: head: remove unnecessary instructions from
 __relocate_kernel

On Sat, Mar 02, 2024 at 02:52:10PM +0000, Puranjay Mohan wrote:
> Formerly, we had to access the RELA and RELR tables via the kernel
> mapping that was being relocated, and so deriving the start and end
> addresses was done by loading __rela_offset in x9 and then adding the
> actual virtual offset of the kernel to it.
> 
> d7bea550279d ("arm64: head: use relative references to the RELA and RELR
> tables") changed this to load the entries via the ID map as we map the
> entire kernel image via the ID map.
> 
> The two instructions for finding the actual virtual offset were left by
> that commit, so remove them now as they are unnecessary and add runtime
> overhead.

These instructions are already gone in linux-next; they're removed by commit:

  734958ef0b5497b1 ("arm64: head: move relocation handling to C code")

.. so this patch isn't necessary.

> Fixes: d7bea550279d ("arm64: head: use relative references to the RELA and RELR tables")

As a heads-up for future patches, while this would be a cleanup it is not a
fix, and wouldn't need a fixes tag.

Mark.

> Signed-off-by: Puranjay Mohan <puranjay12@...il.com>
> ---
>  arch/arm64/kernel/head.S | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index cab7f91949d8..90db196f7c19 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -787,8 +787,6 @@ SYM_FUNC_START_LOCAL(__relocate_kernel)
>  	 */
>  	adr_l	x9, __rela_start
>  	adr_l	x10, __rela_end
> -	mov_q	x11, KIMAGE_VADDR		// default virtual offset
> -	add	x11, x11, x23			// actual virtual offset
>  
>  0:	cmp	x9, x10
>  	b.hs	1f
> 
> base-commit: 5ad3cb0ed525b80c7f66c32b49a68c1f3510bec9
> -- 
> 2.42.0
>