arm64: entry: cleanup sp_el0 manipulation

The kernel stashes the current task struct in sp_el0 so that this can be
acquired consistently/cheaply when required. When we take an exception
from EL0 we have to:

1) stash the original sp_el0 value
2) find the current task
3) update sp_el0 with the current task pointer

Currently steps #1 and #2 occur in one place, and step #3 a while later.
As the value of sp_el0 is immaterial between these points, let's move
them together to make the code clearer and minimize ifdeffery. This
necessitates moving the comment for MDSCR_EL1.SS.

There should be no functional change as a result of this patch.
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>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/entry.S

@@ -167,9 +167,13 @@ alternative_cb_end
 	.if	\el == 0
 	clear_gp_regs
 	mrs	x21, sp_el0
-	ldr_this_cpu	tsk, __entry_task, x20	// Ensure MDSCR_EL1.SS is clear,
-	ldr	x19, [tsk, #TSK_TI_FLAGS]	// since we can unmask debug
-	disable_step_tsk x19, x20		// exceptions when scheduling.
+	ldr_this_cpu	tsk, __entry_task, x20
+	msr	sp_el0, tsk
+
+	// Ensure MDSCR_EL1.SS is clear, since we can unmask debug exceptions
+	// when scheduling.
+	ldr	x19, [tsk, #TSK_TI_FLAGS]
+	disable_step_tsk x19, x20
 
 	apply_ssbd 1, x22, x23
 
@@ -232,13 +236,6 @@ alternative_else_nop_endif
 	str	w21, [sp, #S_SYSCALLNO]
 	.endif
 
-	/*
-	 * Set sp_el0 to current thread_info.
-	 */
-	.if	\el == 0
-	msr	sp_el0, tsk
-	.endif
-
 	/* Save pmr */
 alternative_if ARM64_HAS_IRQ_PRIO_MASKING
 	mrs_s	x20, SYS_ICC_PMR_EL1