powerpc/64s: Fix unnecessary copy to 0 when kernel is booted at addre…

…ss 0

According to the code logic, when the kernel is loaded at 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: b270beb ("powerpc/64s: Run at the kernel virtual address earlier in boot")
Cc: stable@vger.kernel.org # v6.4+
Signed-off-by: Jinglin Wen <jinglin.wen@shingroup.cn>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240620024150.14857-1-jinglin.wen@shingroup.cn

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. */