[PATCH 05/11] arm64: entry: prepare ret_to_user for function call

Mark Rutland
2020-11-26T12:39:03Z

In a subsequent patch ret_to_user will need to make a C function call
(in some configurations) which may clobber x0-x18 at the start of the
finish_ret_to_user block, before enable_step_tsk consumes the flags
loaded into x1.

In preparation for this, let's load the flags into x19, which is
preserved across C function calls. This avoids a redundant reload of the
flags and ensures we operate on a consistent shapshot regardless.

There should be no functional change as a result of this patch. At this
point of the entry/exit paths we only need to preserve x28 (tsk) and the
sp, and x19 is free for this use.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/entry.S | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index b295fb912b12..84aec600eeed 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -774,13 +774,13 @@ SYM_CODE_END(el0_error)
SYM_CODE_START_LOCAL(ret_to_user)
disable_daif
gic_prio_kentry_setup tmp=x3
- ldr x1, [tsk, #TSK_TI_FLAGS]
- and x2, x1, #_TIF_WORK_MASK
+ ldr x19, [tsk, #TSK_TI_FLAGS]
+ and x2, x19, #_TIF_WORK_MASK
cbnz x2, work_pending
finish_ret_to_user:
/* Ignore asynchronous tag check faults in the uaccess routines */
clear_mte_async_tcf
- enable_step_tsk x1, x2
+ enable_step_tsk x19, x2
#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
bl stackleak_erase
#endif
@@ -791,11 +791,12 @@ finish_ret_to_user:
*/
work_pending:
mov x0, sp // 'regs'
+ mov x1, x19
bl do_notify_resume
#ifdef CONFIG_TRACE_IRQFLAGS
bl trace_hardirqs_on // enabled while in userspace
#endif
- ldr x1, [tsk, #TSK_TI_FLAGS] // re-check for single-step
+ ldr x19, [tsk, #TSK_TI_FLAGS] // re-check for single-step
b finish_ret_to_user
SYM_CODE_END(ret_to_user)