[RFC PATCH v2 13/18] arm64: head: record CPU boot mode after enabling the MMU

[Ard Biesheuvel]

In order to avoid having to touch memory with the MMU and caches
disabled, and therefore having to invalidate it from the caches
explicitly, just defer storing the boot mode until after the MMU has
been turned on.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/head.S | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index db315129f15d..ec57a29f3f43 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -81,7 +81,8 @@
 	 * The following callee saved general purpose registers are used on the
 	 * primary lowlevel boot path:
 	 *
-	 *  Register   Scope                      Purpose
+	 *  Register   Scope                                    Purpose
+	 *  x20        primary_entry() .. __primary_switch()    CPU boot mode
 	 *  x21        primary_entry() .. start_kernel()        FDT pointer passed at boot in x0
 	 *  x22        primary_entry() .. start_kernel()        whether we entered with the MMU on
 	 *  x23        __primary_switch() .. relocate_kernel()  physical misalignment/KASLR offset
@@ -91,7 +92,7 @@ SYM_CODE_START(primary_entry)
 	bl	preserve_boot_args
 	bl	create_idmap
 	bl	init_kernel_el			// w0=cpu_boot_mode
-	bl	set_cpu_boot_mode_flag
+	mov	x20, x0
 
 	/*
 	 * The following calls CPU setup code, see arch/arm64/mm/proc.S for
@@ -576,8 +577,6 @@ SYM_FUNC_START_LOCAL(set_cpu_boot_mode_flag)
 	b.ne	1f
 	add	x1, x1, #4
 1:	str	w0, [x1]			// Save CPU boot mode
-	dmb	sy
-	dc	ivac, x1			// Invalidate potentially stale cache line
 	ret
 SYM_FUNC_END(set_cpu_boot_mode_flag)
 
@@ -615,7 +614,7 @@ SYM_DATA_END(__early_cpu_boot_status)
 	 */
 SYM_FUNC_START(secondary_holding_pen)
 	bl	init_kernel_el			// w0=cpu_boot_mode
-	bl	set_cpu_boot_mode_flag
+	mov	x20, x0
 	mrs	x0, mpidr_el1
 	mov_q	x1, MPIDR_HWID_BITMASK
 	and	x0, x0, x1
@@ -633,7 +632,7 @@ SYM_FUNC_END(secondary_holding_pen)
 	 */
 SYM_FUNC_START(secondary_entry)
 	bl	init_kernel_el			// w0=cpu_boot_mode
-	bl	set_cpu_boot_mode_flag
+	mov	x20, x0
 	b	secondary_startup
 SYM_FUNC_END(secondary_entry)
 
@@ -646,6 +645,8 @@ SYM_FUNC_START_LOCAL(secondary_startup)
 	bl	__cpu_setup			// initialise processor
 	adrp	x1, swapper_pg_dir
 	bl	__enable_mmu
+	mov	x0, x20
+	bl	set_cpu_boot_mode_flag
 	ldr	x8, =__secondary_switched
 	br	x8
 SYM_FUNC_END(secondary_startup)
@@ -861,6 +862,9 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 	bl	__enable_mmu
 	bl	clear_page_tables
 
+	mov	x0, x20
+	bl	set_cpu_boot_mode_flag
+
 #ifdef CONFIG_RELOCATABLE
 	adrp	x23, __PHYS_OFFSET
 	and	x23, x23, MIN_KIMG_ALIGN - 1
-- 
2.30.2