Commit 15ad838d281b ("[MIPS] Always do the ARC64_TWIDDLE_PC thing.")
gives the kernel a chance to jump to the address the kernel is linked to.
It can be seen from the old records that this situation is usually a jump
from XPHYS to KSEG0. For example, kdump operation, kexec_start_address is
usually in XPHYS on Loongson64 platform. Jumping from XPHYS to KSEG0 is a
big jump. With the relocation, there may be small jumps. As follows,
+-----------+
| |
|___________|
| |
| _kernel |
| |
|_\0: |
| jr t0 |---+
| la t0, \0 | |
|___________| |
| | |
| | |
|___________| |
| | |
| kernel | |
| | |
|\0: |<--+
| jr t0 |
| la t0, \0 |
| |
|___________|
| |
+-----------+
_kernel is same as kernel, only the loaded address is different. They are
all in KSEG0. For kernel, `jr t0` is right operation and PC will jump to
\0. For _kernel, `jr t0` is dangerous operation and PC will not jump to
_\0 because `la t0, \0` target address is not PC relative.
This patch get the PC address by fixed `1:` (t0) and dynamic ra at
`1:` (t1). t0 determines the PC Region of the address where the kernel is
linked to. t1 determines the offset relative to the PC Region.
Signed-off-by: Jinyang He <hejinyang@xxxxxxxxxxx>
---
arch/mips/kernel/head.S | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)
diff --git a/arch/mips/kernel/head.S b/arch/mips/kernel/head.S
index a25af1d..0186285 100644
--- a/arch/mips/kernel/head.S
+++ b/arch/mips/kernel/head.S
@@ -79,6 +79,9 @@ FEXPORT(__kernel_entry)
j kernel_entry
#endif /* CONFIG_BOOT_RAW */
+#define PC_REGION_SHIFT 28
+#define PC_REGION_MASK (~(1 << PC_REGION_SHIFT - 1))
+
__REF
NESTED(kernel_entry, 16, sp) # kernel entry point
@@ -89,9 +92,17 @@ NESTED(kernel_entry, 16, sp) # kernel entry point
/* We might not get launched at the address the kernel is linked to,
so we jump there. */
- PTR_LA t0, 0f
- jr t0
-0:
+ PTR_LA t0, 1f
+ bal 1f
+1: move t1, ra
+ PTR_LI t2, PC_REGION_MASK
+ and t0, t2
+ PTR_LI t2, ~PC_REGION_MASK
+ and t1, t2
+ or t0, t1
+ PTR_ADDIU t0, 2f-1b
+ jr t0
+2:
PTR_LA t0, __bss_start # clear .bss
LONG_S zero, (t0)
--
2.1.0
