linux-kernel - Re: [PATCH] powerpc: Fixed duplicate copying in the early boot.

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Date: Mon, 17 Jun 2024 21:28:19 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Jinglin Wen <jinglin.wen@...ngroup.cn>, npiggin@...il.com
Cc: christophe.leroy@...roup.eu, naveen.n.rao@...ux.ibm.com,
 masahiroy@...nel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-kernel@...r.kernel.org, jinglin.wen@...ngroup.cn
Subject: Re: [PATCH] powerpc: Fixed duplicate copying in the early boot.

Jinglin Wen <jinglin.wen@...ngroup.cn> writes:
> According to the code logic, when the kernel is loaded to address 0,
> no copying operation should be performed, but it is currently being
> done.
>
> This patch fixes the issue where the kernel code was incorrectly
> duplicated to address 0 when booting from address 0.
>
> Signed-off-by: Jinglin Wen <jinglin.wen@...ngroup.cn>
> ---
>  arch/powerpc/kernel/head_64.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Thanks for the improved change log.

The subject could probably still be clearer, maybe:
  Fix unnecessary copy to 0 when kernel is booted at address 0

Looks like this was introduced by:

  Fixes: b270bebd34e3 ("powerpc/64s: Run at the kernel virtual address earlier in boot")
  Cc: stable@...r.kernel.org # v6.4+

Let me know if you think otherwise.

Just out of interest, how are you hitting this bug? AFAIK none of our
"normal" boot loaders will load the kernel at 0. 

> diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
> index 4690c219bfa4..6c73551bdc50 100644
> --- a/arch/powerpc/kernel/head_64.S
> +++ b/arch/powerpc/kernel/head_64.S
> @@ -647,7 +647,9 @@ __after_prom_start:
>   * Note: This process overwrites the OF exception vectors.
>   */
>  	LOAD_REG_IMMEDIATE(r3, PAGE_OFFSET)
> -	mr.	r4,r26			/* In some cases the loader may  */
> +	tophys(r4,r26)
> +	cmplwi	cr0,r4,0	/* runtime base addr is zero */
> +	mr	r4,r26			/* In some cases the loader may */
>  	beq	9f			/* have already put us at zero */
	
That is a pretty minimal fix, but I think the code would be clearer if
we just compared the source and destination addresses.

Something like the diff below. Can you confirm that works for you.

cheers

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 4690c219bfa4..6ad1435303f9 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -647,8 +647,9 @@ __after_prom_start:
  * Note: This process overwrites the OF exception vectors.
  */
 	LOAD_REG_IMMEDIATE(r3, PAGE_OFFSET)
-	mr.	r4,r26			/* In some cases the loader may  */
-	beq	9f			/* have already put us at zero */
+	mr	r4, r26			// Load the source address into r4
+	cmpld	cr0, r3, r4		// Check if source == dest
+	beq	9f			// If so skip the copy
 	li	r6,0x100		/* Start offset, the first 0x100 */
 					/* bytes were copied earlier.	 */