arm64: entry: prepare ret_to_user for function call

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>
Link: https://lore.kernel.org/r/20201130115950.22492-6-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>

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)