[PATCH v4 15/15] ARM: v7m: enable support for IRQ stacks

[Ard Biesheuvel]

Enable support for IRQ stacks on !MMU, and add the code to the IRQ entry
path to switch to the IRQ stack if not running from it already.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Tested-by: Marc Zyngier <maz at kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin at arm.com> # ARMv7M
---
 arch/arm/Kconfig            |  1 -
 arch/arm/kernel/entry-v7m.S | 17 +++++++++++++++--
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 50ae5286f59b..359a3b85c8b3 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1158,7 +1158,6 @@ config CURRENT_POINTER_IN_TPIDRURO
 
 config IRQSTACKS
 	def_bool y
-	depends on MMU
 	select HAVE_IRQ_EXIT_ON_IRQ_STACK
 	select HAVE_SOFTIRQ_ON_OWN_STACK
 
diff --git a/arch/arm/kernel/entry-v7m.S b/arch/arm/kernel/entry-v7m.S
index 4e0d318b67c6..de8a60363c85 100644
--- a/arch/arm/kernel/entry-v7m.S
+++ b/arch/arm/kernel/entry-v7m.S
@@ -40,11 +40,24 @@ __irq_entry:
 	@ Invoke the IRQ handler
 	@
 	mov	r0, sp
-	stmdb	sp!, {lr}
+	ldr_this_cpu sp, irq_stack_ptr, r1, r2
+
+	@
+	@ If we took the interrupt while running in the kernel, we may already
+	@ be using the IRQ stack, so revert to the original value in that case.
+	@
+	subs	r2, sp, r0		@ SP above bottom of IRQ stack?
+	rsbscs	r2, r2, #THREAD_SIZE	@ ... and below the top?
+	movcs	sp, r0
+
+	push	{r0, lr}		@ preserve LR and original SP
+
 	@ routine called with r0 = struct pt_regs *
 	bl	generic_handle_arch_irq
 
-	pop	{lr}
+	pop	{r0, lr}
+	mov	sp, r0
+
 	@
 	@ Check for any pending work if returning to user
 	@
-- 
2.30.2