[PATCHv2 2/4] arm64: place initial page tables above the kernel

Mark Rutland
2014-05-27T13:18:28Z

Currently we place swapper_pg_dir and idmap_pg_dir below the kernel
image, between PHYS_OFFSET and (PHYS_OFFSET + TEXT_OFFSET). However,
bootloaders may use portions of this memory below the kernel and we do
not parse the memory reservation list until after the MMU has been
enabled. As such we may clobber some memory a bootloader wishes to have
preserved.

To enable the use of all of this memory by bootloaders (when the
required memory reservations are communicated to the kernel) it is
necessary to move our initial page tables elsewhere. As we currently
have an effectively unbound requirement for memory at the end of the
kernel image for .bss, we can place the page tables here.

This patch moves the initial page table to the end of the kernel image,
after the BSS. As they do not consist of any initialised data they will
be stripped from the kernel Image as with the BSS. The BSS clearing
routine is updated to stop at __bss_stop rather than _end so as to not
clobber the page tables, and memory reservations made redundant by the
new organisation are removed.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <lauraa@codeaurora.org>
---
arch/arm64/include/asm/page.h | 9 +++++++++
arch/arm64/kernel/head.S | 28 ++++++++--------------------
arch/arm64/kernel/vmlinux.lds.S | 7 +++++++
arch/arm64/mm/init.c | 12 ++++--------
4 files changed, 28 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
index 46bf666..a6331e6 100644
--- a/arch/arm64/include/asm/page.h
+++ b/arch/arm64/include/asm/page.h
@@ -31,6 +31,15 @@
/* We do define AT_SYSINFO_EHDR but don't use the gate mechanism */
#define __HAVE_ARCH_GATE_AREA 1

+/*
+ * The idmap and swapper page tables need some space reserved in the kernel
+ * image. The idmap only requires a pgd and a next level table to (section) map
+ * the kernel, while the swapper also maps the FDT and requires an additional
+ * table to map an early UART. See __create_page_tables for more information.
+ */
+#define SWAPPER_DIR_SIZE (3 * PAGE_SIZE)
+#define IDMAP_DIR_SIZE (2 * PAGE_SIZE)
+
#ifndef __ASSEMBLY__

#ifdef CONFIG_ARM64_64K_PAGES
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index dc3ee59..36ecee2 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -35,29 +35,17 @@
#include <asm/page.h>
#include <asm/virt.h>

-/*
- * swapper_pg_dir is the virtual address of the initial page table. We place
- * the page tables 3 * PAGE_SIZE below KERNEL_RAM_VADDR. The idmap_pg_dir has
- * 2 pages and is placed below swapper_pg_dir.
- */
#define KERNEL_RAM_VADDR (PAGE_OFFSET + TEXT_OFFSET)

#if (KERNEL_RAM_VADDR & 0xfffff) != 0x80000
#error KERNEL_RAM_VADDR must start at 0xXXX80000
#endif

-#define SWAPPER_DIR_SIZE (3 * PAGE_SIZE)
-#define IDMAP_DIR_SIZE (2 * PAGE_SIZE)
-
- .globl swapper_pg_dir
- .equ swapper_pg_dir, KERNEL_RAM_VADDR - SWAPPER_DIR_SIZE
-
- .globl idmap_pg_dir
- .equ idmap_pg_dir, swapper_pg_dir - IDMAP_DIR_SIZE
-
- .macro pgtbl, ttb0, ttb1, phys
- add \ttb1, \phys, #TEXT_OFFSET - SWAPPER_DIR_SIZE
- sub \ttb0, \ttb1, #IDMAP_DIR_SIZE
+ .macro pgtbl, ttb0, ttb1, virt_to_phys
+ ldr \ttb1, =swapper_pg_dir
+ ldr \ttb0, =idmap_pg_dir
+ add \ttb1, \ttb1, \virt_to_phys
+ add \ttb0, \ttb0, \virt_to_phys
.endm

#ifdef CONFIG_ARM64_64K_PAGES
@@ -416,7 +404,7 @@ ENTRY(secondary_startup)
mov x23, x0 // x23=current cpu_table
cbz x23, __error_p // invalid processor (x23=0)?

- pgtbl x25, x26, x24 // x25=TTBR0, x26=TTBR1
+ pgtbl x25, x26, x28 // x25=TTBR0, x26=TTBR1
ldr x12, [x23, #CPU_INFO_SETUP]
add x12, x12, x28 // __virt_to_phys
blr x12 // initialise processor
@@ -530,7 +518,7 @@ ENDPROC(__calc_phys_offset)
* - pgd entry for fixed mappings (TTBR1)
*/
__create_page_tables:
- pgtbl x25, x26, x24 // idmap_pg_dir and swapper_pg_dir addresses
+ pgtbl x25, x26, x28 // idmap_pg_dir and swapper_pg_dir addresses
mov x27, lr

/*
@@ -619,7 +607,7 @@ ENDPROC(__create_page_tables)
__switch_data:
.quad __mmap_switched
.quad __bss_start // x6
- .quad _end // x7
+ .quad __bss_stop // x7
.quad processor_id // x4
.quad __fdt_pointer // x5
.quad memstart_addr // x6
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 4ba7a55..51258bc 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -104,6 +104,13 @@ SECTIONS
_edata = .;

BSS_SECTION(0, 0, 0)
+
+ . = ALIGN(PAGE_SIZE);
+ idmap_pg_dir = .;
+ . += IDMAP_DIR_SIZE;
+ swapper_pg_dir = .;
+ . += SWAPPER_DIR_SIZE;
+
_end = .;

STABS_DEBUG
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 51d5352..cc3339d 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -128,20 +128,16 @@ void __init arm64_memblock_init(void)
{
u64 *reserve_map, base, size;

- /* Register the kernel text, kernel data and initrd with memblock */
+ /*
+ * Register the kernel text, kernel data, initrd, and initial
+ * pagetables with memblock.
+ */
memblock_reserve(__pa(_text), _end - _text);
#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start)
memblock_reserve(__virt_to_phys(initrd_start), initrd_end - initrd_start);
#endif

- /*
- * Reserve the page tables. These are already in use,
- * and can only be in node 0.
- */
- memblock_reserve(__pa(swapper_pg_dir), SWAPPER_DIR_SIZE);
- memblock_reserve(__pa(idmap_pg_dir), IDMAP_DIR_SIZE);
-
/* Reserve the dtb region */
memblock_reserve(virt_to_phys(initial_boot_params),
be32_to_cpu(initial_boot_params->totalsize));
--
1.9.1