Hi Michael Ellerman,

Michael Ellerman [off-list ref] writes:

Jinglin Wen [off-list ref] writes:

quoted

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 <redacted>
---
 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

Thanks for your feedback, I will revise my subject.

Looks like this was introduced by:

  Fixes: b270bebd34e3 ("powerpc/64s: Run at the kernel virtual address earlier in boot")
  Cc: stable@vger.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. 

quoted

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

As for how I discovered this bug, we use zImage.epapr for emulation, which 
loads vmlinux.bin at address 0. When vmlinux.bin is relatively large, I 
found that the boot time of Linux 6.6 is much slower compared to Linux 5.10.108. 
I discovered this issue while comparing the code between the two versions.

Indeed, your code looks much clearer. I will make the following modifications 
based on your code:

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 4690c219bfa4..751181dfb897 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 */
+       cmpd    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.    */ 

Thanks,

Jinglin Wen