[PATCH] [RFC] aarch64: scs: reload shadow call stack in user exception entry

[Dan Li <ashimida@linux.alibaba.com>]

When el0 exception occurs, kernel_entry/exit will load/save tsk->scs_sp
to ensure scs working properly. AFAIK, the SCS offset should always be
0 at this time.

Is it reasonable to reload x18 to scs_base directly in kernel_entry
here, or am I missing something?

Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
---
 arch/arm64/include/asm/scs.h | 7 +++++++
 arch/arm64/kernel/entry.S    | 3 +--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
index 8297bccf0784..2bc0d0575e75 100644
--- a/arch/arm64/include/asm/scs.h
+++ b/arch/arm64/include/asm/scs.h
@@ -9,6 +9,10 @@
 #ifdef CONFIG_SHADOW_CALL_STACK
 	scs_sp	.req	x18
 
+	.macro scs_reload tsk
+	ldr	scs_sp, [\tsk, #TSK_TI_SCS_BASE]
+	.endm
+
 	.macro scs_load tsk
 	ldr	scs_sp, [\tsk, #TSK_TI_SCS_SP]
 	.endm
@@ -17,6 +21,9 @@
 	str	scs_sp, [\tsk, #TSK_TI_SCS_SP]
 	.endm
 #else
+	.macro scs_reload tsk
+	.endm
+
 	.macro scs_load tsk
 	.endm
 
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index bc6d5a970a13..57547a3e4f7c 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -265,7 +265,7 @@ alternative_if ARM64_HAS_ADDRESS_AUTH
 alternative_else_nop_endif
 1:
 
-	scs_load tsk
+	scs_reload tsk
 	.else
 	add	x21, sp, #PT_REGS_SIZE
 	get_current_task tsk
@@ -365,7 +365,6 @@ alternative_if ARM64_WORKAROUND_845719
 alternative_else_nop_endif
 #endif
 3:
-	scs_save tsk
 
 	/* Ignore asynchronous tag check faults in the uaccess routines */
 	ldr	x0, [tsk, THREAD_SCTLR_USER]
-- 
2.17.1