[PATCH v2] powerpc: Fix unnecessary copy to 0 when kernel is booted at address 0.

[Jinglin Wen]

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.

Fixes: b270bebd34e3 ("powerpc/64s: Run at the kernel virtual address earlier in boot")
Signed-off-by: Jinglin Wen <jinglin.wen at shingroup.cn>
Suggested-by: Michael Ellerman <mpe at ellerman.id.au>
Cc: <stable at vger.kernel.org>
---

v2:
  - According to 87le336c6k.fsf at mail.lhotse, improve this patch.
v1:
  - 20240617023509.5674-1-jinglin.wen at shingroup.cn

 arch/powerpc/kernel/head_64.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 4690c219bfa4..63432a33ec49 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 virtual source address into r4 */
+	cmpld	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.	 */
 
-- 
2.25.1