arm64: unmap idmap earlier

During boot we leave the idmap in place until paging_init, as we
previously had to wait for the zero page to become allocated and
accessible.

Now that we have a statically-allocated zero page, we can uninstall the
idmap much earlier in the boot process, making it far easier to spot
accidental use of physical addresses. This also brings the cold boot
path in line with the secondary boot path.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/setup.c

@@ -314,6 +314,12 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	local_async_enable();
 
+	/*
+	 * TTBR0 is only used for the identity mapping at this stage. Make it
+	 * point to zero page to avoid speculatively fetching new entries.
+	 */
+	cpu_uninstall_idmap();
+
 	efi_init();
 	arm64_memblock_init();
 

arch/arm64/mm/mmu.c

@@ -463,12 +463,6 @@ void __init paging_init(void)
 	fixup_executable();
 
 	bootmem_init();
-
-	/*
-	 * TTBR0 is only used for the identity mapping at this stage. Make it
-	 * point to zero page to avoid speculatively fetching new entries.
-	 */
-	cpu_uninstall_idmap();
 }
 
 /*