Subject: [PATCH v2 3/5] ARM: Disable jump-label on PREEMPT_RT
From: Thomas Gleixner <[email-masked]>
Date: Tue, 10 Dec 2024 17:05:53 +0100

jump-labels are used to efficiently switch between two possible code
paths. To achieve this, stop_machine() is used to keep the CPU in a
known state while the opcode is modified. The usage of stop_machine()
here leads to large latency spikes which can be observed on PREEMPT_RT.

Jump labels may change the target during runtime and are not restricted
to debug or "configuration/ setup" part of a PREEMPT_RT system where
high latencies could be defined as acceptable.

On 64-bit Arm, it is possible to use jump labels without the
stop_machine() call, which architecturally provides a way to atomically
change one 32-bit instruction word while keeping maintaining consistency,
but this is not generally the case on 32-bit, in particular in thumb2
mode.

Disable jump-label support on a PREEMPT_RT system when SMP is enabled.

[bigeasy: Patch description.]
[arnd: add !SMP case, extend changelog]

Signed-off-by: Thomas Gleixner <[email-masked]>
Cc: Mark Rutland <[email-masked]>
Cc: Ard Biesheuvel <[email-masked]>
Signed-off-by: Arnd Bergmann <[email-masked]>
Reviewed-by: Linus Walleij <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 arch/arm/Kconfig |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -80,7 +80,7 @@ config ARM
 	select HAS_IOPORT
 	select HAVE_ARCH_AUDITSYSCALL if AEABI && !OABI_COMPAT
 	select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
-	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU && (!PREEMPT_RT || !SMP)
 	select HAVE_ARCH_KFENCE if MMU && !XIP_KERNEL
 	select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
 	select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL