arm64: head: record the MMU state at primary entry

Prepare for being able to deal with primary entry with the MMU and
caches enabled, by recording whether or not we entered with the MMU on
in register x19 and in a global variable. (Note that setting this
variable to '1' does not require cache invalidation, nor is it required
for storing the bootargs in that case, so omit the cache maintenance).

Since boot with the MMU and caches enabled is not permitted by the bare
metal boot protocol, ensure that a diagnostic is emitted and a taint bit
set if the MMU was found to be enabled on a non-EFI boot, and panic()
once the console is likely to be up. We will make an exception for EFI
boot later, which has strict requirements for the mapping of system
memory, permitting us to relax the boot protocol and hand over from the
EFI stub to the core kernel with MMU and caches left enabled.

While at it, add 'pre_disable_mmu_workaround' macro invocations to
init_kernel_el, as its manipulation of SCTLR_ELx may amount to disabling
of the MMU after subsequent patches.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230111102236.1430401-4-ardb@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/head.S

@@ -77,6 +77,7 @@
 	 * primary lowlevel boot path:
 	 *
 	 *  Register   Scope                      Purpose
+	 *  x19        primary_entry() .. start_kernel()        whether we entered with the MMU on
 	 *  x20        primary_entry() .. __primary_switch()    CPU boot mode
 	 *  x21        primary_entry() .. start_kernel()        FDT pointer passed at boot in x0
 	 *  x22        create_idmap() .. start_kernel()         ID map VA of the DT blob
@@ -86,6 +87,7 @@
 	 *  x28        create_idmap()                           callee preserved temp register
 	 */
 SYM_CODE_START(primary_entry)
+	bl	record_mmu_state
 	bl	preserve_boot_args
 	bl	init_kernel_el			// w0=cpu_boot_mode
 	mov	x20, x0
@@ -109,6 +111,18 @@ SYM_CODE_START(primary_entry)
 	b	__primary_switch
 SYM_CODE_END(primary_entry)
 
+SYM_CODE_START_LOCAL(record_mmu_state)
+	mrs	x19, CurrentEL
+	cmp	x19, #CurrentEL_EL2
+	mrs	x19, sctlr_el1
+	b.ne	0f
+	mrs	x19, sctlr_el2
+0:	tst	x19, #SCTLR_ELx_C		// Z := (C == 0)
+	and	x19, x19, #SCTLR_ELx_M		// isolate M bit
+	csel	x19, xzr, x19, eq		// clear x19 if Z
+	ret
+SYM_CODE_END(record_mmu_state)
+
 /*
  * Preserve the arguments passed by the bootloader in x0 .. x3
  */
@@ -119,11 +133,14 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
 	stp	x21, x1, [x0]			// x0 .. x3 at kernel entry
 	stp	x2, x3, [x0, #16]
 
+	cbnz	x19, 0f				// skip cache invalidation if MMU is on
 	dmb	sy				// needed before dc ivac with
 						// MMU off
 
 	add	x1, x0, #0x20			// 4 x 8 bytes
 	b	dcache_inval_poc		// tail call
+0:	str_l   x19, mmu_enabled_at_boot, x0
+	ret
 SYM_CODE_END(preserve_boot_args)
 
 SYM_FUNC_START_LOCAL(clear_page_tables)
@@ -497,6 +514,7 @@ SYM_FUNC_START(init_kernel_el)
 
 SYM_INNER_LABEL(init_el1, SYM_L_LOCAL)
 	mov_q	x0, INIT_SCTLR_EL1_MMU_OFF
+	pre_disable_mmu_workaround
 	msr	sctlr_el1, x0
 	isb
 	mov_q	x0, INIT_PSTATE_EL1
@@ -529,11 +547,13 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
 	cbz	x0, 1f
 
 	/* Set a sane SCTLR_EL1, the VHE way */
+	pre_disable_mmu_workaround
 	msr_s	SYS_SCTLR_EL12, x1
 	mov	x2, #BOOT_CPU_FLAG_E2H
 	b	2f
 
 1:
+	pre_disable_mmu_workaround
 	msr	sctlr_el1, x1
 	mov	x2, xzr
 2:

arch/arm64/kernel/setup.c

@@ -58,6 +58,7 @@ static int num_standard_resources;
 static struct resource *standard_resources;
 
 phys_addr_t __fdt_pointer __initdata;
+u64 mmu_enabled_at_boot __initdata;
 
 /*
  * Standard memory resources
@@ -332,8 +333,12 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	xen_early_init();
 	efi_init();
 
-	if (!efi_enabled(EFI_BOOT) && ((u64)_text % MIN_KIMG_ALIGN) != 0)
+	if (!efi_enabled(EFI_BOOT)) {
+		if ((u64)_text % MIN_KIMG_ALIGN)
 			pr_warn(FW_BUG "Kernel image misaligned at boot, please fix your bootloader!");
+		WARN_TAINT(mmu_enabled_at_boot, TAINT_FIRMWARE_WORKAROUND,
+			   FW_BUG "Booted with MMU enabled!");
+	}
 
 	arm64_memblock_init();
 
@@ -442,3 +447,11 @@ static int __init register_arm64_panic_block(void)
 	return 0;
 }
 device_initcall(register_arm64_panic_block);
+
+static int __init check_mmu_enabled_at_boot(void)
+{
+	if (!efi_enabled(EFI_BOOT) && mmu_enabled_at_boot)
+		panic("Non-EFI boot detected with MMU and caches enabled");
+	return 0;
+}
+device_initcall_sync(check_mmu_enabled_at_boot);