[PATCH 3/7] arm64: asm: Introduce test_sp_overflow macro

[Kalesh Singh]

From: Quentin Perret <qperret at google.com>

The asm entry code in the kernel uses a trick to check if VMAP'd stacks
have overflowed by aligning them at THREAD_SHIFT * 2 granularity and
checking the SP's THREAD_SHIFT bit.

Protected KVM will soon make use of a similar trick to detect stack
overflows, so factor out the asm code in a re-usable macro.

Signed-off-by: Quentin Perret <qperret at google.com>
[Kalesh - Resolve minor conflicts]
Signed-off-by: Kalesh Singh <kaleshsingh at google.com>
---
 arch/arm64/include/asm/assembler.h | 11 +++++++++++
 arch/arm64/kernel/entry.S          |  9 ++-------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index e8bd0af0141c..ad40eb0eee83 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -850,4 +850,15 @@ alternative_endif
 
 #endif /* GNU_PROPERTY_AARCH64_FEATURE_1_DEFAULT */
 
+/*
+ * Test whether the SP has overflowed, without corrupting a GPR.
+ */
+.macro test_sp_overflow shift, label
+	add	sp, sp, x0			// sp' = sp + x0
+	sub	x0, sp, x0			// x0' = sp' - x0 = (sp + x0) - x0 = sp
+	tbnz	x0, #\shift, \label
+	sub	x0, sp, x0			// x0'' = sp' - x0' = (sp + x0) - sp = x0
+	sub	sp, sp, x0			// sp'' = sp' - x0 = (sp + x0) - x0 = sp
+.endm
+
 #endif	/* __ASM_ASSEMBLER_H */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 772ec2ecf488..2632bc47b348 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -53,16 +53,11 @@ alternative_else_nop_endif
 	sub	sp, sp, #PT_REGS_SIZE
 #ifdef CONFIG_VMAP_STACK
 	/*
-	 * Test whether the SP has overflowed, without corrupting a GPR.
 	 * Task and IRQ stacks are aligned so that SP & (1 << THREAD_SHIFT)
 	 * should always be zero.
 	 */
-	add	sp, sp, x0			// sp' = sp + x0
-	sub	x0, sp, x0			// x0' = sp' - x0 = (sp + x0) - x0 = sp
-	tbnz	x0, #THREAD_SHIFT, 0f
-	sub	x0, sp, x0			// x0'' = sp' - x0' = (sp + x0) - sp = x0
-	sub	sp, sp, x0			// sp'' = sp' - x0 = (sp + x0) - x0 = sp
-	b	el\el\ht\()_\regsize\()_\label
+	test_sp_overflow THREAD_SHIFT, 0f
+	b       el\el\ht\()_\regsize\()_\label
 
 0:
 	/*
-- 
2.35.1.265.g69c8d7142f-goog