From 3b4442b6e7810053d4e0c00723e2d93c0257e118 Mon Sep 17 00:00:00 2001
From: chaoskagami <chaos.kagami@gmail.com>
Date: Fri, 14 Oct 2016 19:58:27 -0400
Subject: [PATCH] Disable IRQ/FIQ while in svcBackdoor

---
 external/bits/backdoor.s | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/external/bits/backdoor.s b/external/bits/backdoor.s
index 9a22f67..17c2f23 100644
--- a/external/bits/backdoor.s
+++ b/external/bits/backdoor.s
@@ -1,16 +1,36 @@
-// This is svcBackdoor's code from earlier FIRMs
-// Luckily, no ARM9/ARM11 specific instructions are used here.
-// It can just be assembled via ARM9 gas.
+// This is a "backdoor" supervisor call we install.
+// Roughly, this is based on nintendo's, but there are a few defecits in their code.
+
+// Namely, Nintendo's will crash ARM11 kernel if an IRQ/FIQ is hit while in the call,
+// so we play it safe and disable them in cpsr while in backdoor.
+
 .section .text
 .global _start
 _start:
+	// Disable FIQ and IRQ.
+	mrs r1, cpsr
+	orr r1, #0xc0
+	msr cpsr_c, r1
+
+	// Save
 	bic r1, sp, #0xff
 	orr r1, r1, #0xf00
 	add r1, r1, #0x28
 	ldr r2, [r1]
 	stmdb r2!, {sp, lr}
 	mov sp, r2
+
+	// Call user function.
 	blx r0
+
+	// Renable FIQ and IRQ.
+	mrs r1, cpsr
+	bic r1, #0xc0
+	msr cpsr_c, r1
+
+	// Restore registers.
 	pop {r0, r1}
 	mov sp, r0
+
+	// Return.
 	bx r1
-- 
2.39.5