Re: [PATCH 2/8] arm64: use fixmap region for permanent FDT mapping

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On 1 June 2015 at 11:53, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> Hi Ard,
>
> Sorry for the silence on this.
>
> On Mon, May 11, 2015 at 07:41:54AM +0100, Ard Biesheuvel wrote:
>> Currently, the FDT blob needs to be in the same 512 MB region as
>> the kernel, so that it can be mapped into the kernel virtual memory
>> space very early on using a minimal set of statically allocated
>> translation tables.
>>
>> Now that we have early fixmap support, we can relax this restriction,
>> by moving the permanent FDT mapping to the fixmap region instead.
>> This way, the FDT blob may be anywhere in memory.
>>
>> This also moves the vetting of the FDT to mmu.c, since the early
>> init code in head.S does not handle mapping of the FDT anymore.
>> At the same time, fix up some comments in head.S that have gone stale.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>
> The patch looks good to me. I've just given this some testing loading
> DTBs at weird offsets and large disances from the kernel, and that seems
> to work, so:
>
> Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
> Tested-by: Mark Rutland <mark.rutland@xxxxxxx>
>

Thank you.

I noticed that I forgot to make the Documentation change you requested
last time around, so I will roll that into the next version (with your
tags).

-- 
Ard.


>> ---
>>  Documentation/arm64/booting.txt | 13 ++++----
>>  arch/arm64/include/asm/boot.h   | 14 +++++++++
>>  arch/arm64/include/asm/fixmap.h | 15 ++++++++++
>>  arch/arm64/include/asm/mmu.h    |  1 +
>>  arch/arm64/kernel/head.S        | 39 +-----------------------
>>  arch/arm64/kernel/setup.c       | 30 +++++++------------
>>  arch/arm64/mm/Makefile          |  2 ++
>>  arch/arm64/mm/init.c            |  1 -
>>  arch/arm64/mm/mmu.c             | 66 +++++++++++++++++++++++++++++++++++++++++
>>  9 files changed, 117 insertions(+), 64 deletions(-)
>>  create mode 100644 arch/arm64/include/asm/boot.h
>>
>> diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
>> index f3c05b5f9f08..53f18e13d51c 100644
>> --- a/Documentation/arm64/booting.txt
>> +++ b/Documentation/arm64/booting.txt
>> @@ -45,11 +45,14 @@ sees fit.)
>>
>>  Requirement: MANDATORY
>>
>> -The device tree blob (dtb) must be placed on an 8-byte boundary within
>> -the first 512 megabytes from the start of the kernel image and must not
>> -cross a 2-megabyte boundary. This is to allow the kernel to map the
>> -blob using a single section mapping in the initial page tables.
>> -
>> +The device tree blob (dtb) must be placed on an 8-byte boundary and must
>> +not exceed 2 megabytes in size. Since the dtb will be mapped cacheable using
>> +blocks of up to 2 megabytes in size, it should not be placed within 2 megabytes
>> +of memreserves or other special carveouts that may be mapped with non-matching
>> +attributes.
>> +
>> +NOTE: versions prior to v4.2 also require that the DTB be placed within
>> +the 512 MB region starting at text_offset bytes below the kernel Image.
>>
>>  3. Decompress the kernel image
>>  ------------------------------
>> diff --git a/arch/arm64/include/asm/boot.h b/arch/arm64/include/asm/boot.h
>> new file mode 100644
>> index 000000000000..81151b67b26b
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/boot.h
>> @@ -0,0 +1,14 @@
>> +
>> +#ifndef __ASM_BOOT_H
>> +#define __ASM_BOOT_H
>> +
>> +#include <asm/sizes.h>
>> +
>> +/*
>> + * arm64 requires the DTB to be 8 byte aligned and
>> + * not exceed 2MB in size.
>> + */
>> +#define MIN_FDT_ALIGN          8
>> +#define MAX_FDT_SIZE           SZ_2M
>> +
>> +#endif
>> diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
>> index 95e6b6dcbe37..c0739187a920 100644
>> --- a/arch/arm64/include/asm/fixmap.h
>> +++ b/arch/arm64/include/asm/fixmap.h
>> @@ -17,6 +17,7 @@
>>
>>  #ifndef __ASSEMBLY__
>>  #include <linux/kernel.h>
>> +#include <asm/boot.h>
>>  #include <asm/page.h>
>>
>>  /*
>> @@ -32,6 +33,20 @@
>>   */
>>  enum fixed_addresses {
>>         FIX_HOLE,
>> +
>> +       /*
>> +        * Reserve a virtual window for the FDT that is 2 MB larger than the
>> +        * maximum supported size, and put it at the top of the fixmap region.
>> +        * The additional space ensures that any FDT that does not exceed
>> +        * MAX_FDT_SIZE can be mapped regardless of whether it crosses any
>> +        * 2 MB alignment boundaries.
>> +        *
>> +        * Keep this at the top so it remains 2 MB aligned.
>> +        */
>> +#define FIX_FDT_SIZE           (MAX_FDT_SIZE + SZ_2M)
>> +       FIX_FDT_END,
>> +       FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
>> +
>>         FIX_EARLYCON_MEM_BASE,
>>         FIX_TEXT_POKE0,
>>         __end_of_permanent_fixed_addresses,
>> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
>> index 3d311761e3c2..79fcfb048884 100644
>> --- a/arch/arm64/include/asm/mmu.h
>> +++ b/arch/arm64/include/asm/mmu.h
>> @@ -34,5 +34,6 @@ extern void init_mem_pgprot(void);
>>  extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
>>                                unsigned long virt, phys_addr_t size,
>>                                pgprot_t prot);
>> +extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
>>
>>  #endif
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index 19f915e8f6e0..30cffc5e7402 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -237,8 +237,6 @@ ENTRY(stext)
>>         bl      el2_setup                       // Drop to EL1, w20=cpu_boot_mode
>>         adrp    x24, __PHYS_OFFSET
>>         bl      set_cpu_boot_mode_flag
>> -
>> -       bl      __vet_fdt
>>         bl      __create_page_tables            // x25=TTBR0, x26=TTBR1
>>         /*
>>          * The following calls CPU setup code, see arch/arm64/mm/proc.S for
>> @@ -270,24 +268,6 @@ preserve_boot_args:
>>  ENDPROC(preserve_boot_args)
>>
>>  /*
>> - * Determine validity of the x21 FDT pointer.
>> - * The dtb must be 8-byte aligned and live in the first 512M of memory.
>> - */
>> -__vet_fdt:
>> -       tst     x21, #0x7
>> -       b.ne    1f
>> -       cmp     x21, x24
>> -       b.lt    1f
>> -       mov     x0, #(1 << 29)
>> -       add     x0, x0, x24
>> -       cmp     x21, x0
>> -       b.ge    1f
>> -       ret
>> -1:
>> -       mov     x21, #0
>> -       ret
>> -ENDPROC(__vet_fdt)
>> -/*
>>   * Macro to create a table entry to the next page.
>>   *
>>   *     tbl:    page table address
>> @@ -348,8 +328,7 @@ ENDPROC(__vet_fdt)
>>   * required to get the kernel running. The following sections are required:
>>   *   - identity mapping to enable the MMU (low address, TTBR0)
>>   *   - first few MB of the kernel linear mapping to jump to once the MMU has
>> - *     been enabled, including the FDT blob (TTBR1)
>> - *   - pgd entry for fixed mappings (TTBR1)
>> + *     been enabled
>>   */
>>  __create_page_tables:
>>         adrp    x25, idmap_pg_dir
>> @@ -439,22 +418,6 @@ __create_page_tables:
>>         create_block_map x0, x7, x3, x5, x6
>>
>>         /*
>> -        * Map the FDT blob (maximum 2MB; must be within 512MB of
>> -        * PHYS_OFFSET).
>> -        */
>> -       mov     x3, x21                         // FDT phys address
>> -       and     x3, x3, #~((1 << 21) - 1)       // 2MB aligned
>> -       mov     x6, #PAGE_OFFSET
>> -       sub     x5, x3, x24                     // subtract PHYS_OFFSET
>> -       tst     x5, #~((1 << 29) - 1)           // within 512MB?
>> -       csel    x21, xzr, x21, ne               // zero the FDT pointer
>> -       b.ne    1f
>> -       add     x5, x5, x6                      // __va(FDT blob)
>> -       add     x6, x5, #1 << 21                // 2MB for the FDT blob
>> -       sub     x6, x6, #1                      // inclusive range
>> -       create_block_map x0, x7, x3, x5, x6
>> -1:
>> -       /*
>>          * Since the page tables have been populated with non-cacheable
>>          * accesses (MMU disabled), invalidate the idmap and swapper page
>>          * tables again to remove any speculatively loaded cache lines.
>> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
>> index 74753132c3ac..770ae9dc5a75 100644
>> --- a/arch/arm64/kernel/setup.c
>> +++ b/arch/arm64/kernel/setup.c
>> @@ -105,18 +105,6 @@ static struct resource mem_res[] = {
>>  #define kernel_code mem_res[0]
>>  #define kernel_data mem_res[1]
>>
>> -void __init early_print(const char *str, ...)
>> -{
>> -       char buf[256];
>> -       va_list ap;
>> -
>> -       va_start(ap, str);
>> -       vsnprintf(buf, sizeof(buf), str, ap);
>> -       va_end(ap);
>> -
>> -       printk("%s", buf);
>> -}
>> -
>>  /*
>>   * The recorded values of x0 .. x3 upon kernel entry.
>>   */
>> @@ -326,12 +314,14 @@ static void __init setup_processor(void)
>>
>>  static void __init setup_machine_fdt(phys_addr_t dt_phys)
>>  {
>> -       if (!dt_phys || !early_init_dt_scan(phys_to_virt(dt_phys))) {
>> -               early_print("\n"
>> -                       "Error: invalid device tree blob at physical address 0x%p (virtual address 0x%p)\n"
>> -                       "The dtb must be 8-byte aligned and passed in the first 512MB of memory\n"
>> -                       "\nPlease check your bootloader.\n",
>> -                       dt_phys, phys_to_virt(dt_phys));
>> +       void *dt_virt = fixmap_remap_fdt(dt_phys);
>> +
>> +       if (!dt_virt || !early_init_dt_scan(dt_virt)) {
>> +               pr_crit("\n"
>> +                       "Error: invalid device tree blob at physical address %pa (virtual address 0x%p)\n"
>> +                       "The dtb must be 8-byte aligned and must not exceed 2 MB in size\n"
>> +                       "\nPlease check your bootloader.",
>> +                       &dt_phys, dt_virt);
>>
>>                 while (true)
>>                         cpu_relax();
>> @@ -374,8 +364,6 @@ void __init setup_arch(char **cmdline_p)
>>  {
>>         setup_processor();
>>
>> -       setup_machine_fdt(__fdt_pointer);
>> -
>>         init_mm.start_code = (unsigned long) _text;
>>         init_mm.end_code   = (unsigned long) _etext;
>>         init_mm.end_data   = (unsigned long) _edata;
>> @@ -386,6 +374,8 @@ void __init setup_arch(char **cmdline_p)
>>         early_fixmap_init();
>>         early_ioremap_init();
>>
>> +       setup_machine_fdt(__fdt_pointer);
>> +
>>         parse_early_param();
>>
>>         /*
>> diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
>> index 773d37a14039..9d84feb41a16 100644
>> --- a/arch/arm64/mm/Makefile
>> +++ b/arch/arm64/mm/Makefile
>> @@ -4,3 +4,5 @@ obj-y                           := dma-mapping.o extable.o fault.o init.o \
>>                                    context.o proc.o pageattr.o
>>  obj-$(CONFIG_HUGETLB_PAGE)     += hugetlbpage.o
>>  obj-$(CONFIG_ARM64_PTDUMP)     += dump.o
>> +
>> +CFLAGS_mmu.o                   := -I$(srctree)/scripts/dtc/libfdt/
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 89a05f467ffb..597831bdddf3 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -170,7 +170,6 @@ void __init arm64_memblock_init(void)
>>                 memblock_reserve(__virt_to_phys(initrd_start), initrd_end - initrd_start);
>>  #endif
>>
>> -       early_init_fdt_reserve_self();
>>         early_init_fdt_scan_reserved_mem();
>>
>>         /* 4GB maximum for 32-bit only capable devices */
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 5b8b664422d3..82d3435bf14f 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -21,6 +21,7 @@
>>  #include <linux/kernel.h>
>>  #include <linux/errno.h>
>>  #include <linux/init.h>
>> +#include <linux/libfdt.h>
>>  #include <linux/mman.h>
>>  #include <linux/nodemask.h>
>>  #include <linux/memblock.h>
>> @@ -643,3 +644,68 @@ void __set_fixmap(enum fixed_addresses idx,
>>                 flush_tlb_kernel_range(addr, addr+PAGE_SIZE);
>>         }
>>  }
>> +
>> +void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
>> +{
>> +       const u64 dt_virt_base = __fix_to_virt(FIX_FDT);
>> +       pgprot_t prot = PAGE_KERNEL | PTE_RDONLY;
>> +       int granularity, size, offset;
>> +       void *dt_virt;
>> +
>> +       /*
>> +        * Check whether the physical FDT address is set and meets the minimum
>> +        * alignment requirement. Since we are relying on MIN_FDT_ALIGN to be
>> +        * at least 8 bytes so that we can always access the size field of the
>> +        * FDT header after mapping the first chunk, double check here if that
>> +        * is indeed the case.
>> +        */
>> +       BUILD_BUG_ON(MIN_FDT_ALIGN < 8);
>> +       if (!dt_phys || dt_phys % MIN_FDT_ALIGN)
>> +               return NULL;
>> +
>> +       /*
>> +        * Make sure that the FDT region can be mapped without the need to
>> +        * allocate additional translation table pages, so that it is safe
>> +        * to call create_mapping() this early.
>> +        *
>> +        * On 64k pages, the FDT will be mapped using PTEs, so we need to
>> +        * be in the same PMD as the rest of the fixmap.
>> +        * On 4k pages, we'll use section mappings for the FDT so we only
>> +        * have to be in the same PUD.
>> +        */
>> +       BUILD_BUG_ON(dt_virt_base % SZ_2M);
>> +
>> +       if (IS_ENABLED(CONFIG_ARM64_64K_PAGES)) {
>> +               BUILD_BUG_ON(__fix_to_virt(FIX_FDT_END) >> PMD_SHIFT !=
>> +                            __fix_to_virt(FIX_BTMAP_BEGIN) >> PMD_SHIFT);
>> +
>> +               granularity = PAGE_SIZE;
>> +       } else {
>> +               BUILD_BUG_ON(__fix_to_virt(FIX_FDT_END) >> PUD_SHIFT !=
>> +                            __fix_to_virt(FIX_BTMAP_BEGIN) >> PUD_SHIFT);
>> +
>> +               granularity = PMD_SIZE;
>> +       }
>> +
>> +       offset = dt_phys % granularity;
>> +       dt_virt = (void *)dt_virt_base + offset;
>> +
>> +       /* map the first chunk so we can read the size from the header */
>> +       create_mapping(round_down(dt_phys, granularity), dt_virt_base,
>> +                      granularity, prot);
>> +
>> +       if (fdt_check_header(dt_virt) != 0)
>> +               return NULL;
>> +
>> +       size = fdt_totalsize(dt_virt);
>> +       if (size > MAX_FDT_SIZE)
>> +               return NULL;
>> +
>> +       if (offset + size > granularity)
>> +               create_mapping(round_down(dt_phys, granularity), dt_virt_base,
>> +                              round_up(offset + size, granularity), prot);
>> +
>> +       memblock_reserve(dt_phys, size);
>> +
>> +       return dt_virt;
>> +}
>> --
>> 1.9.1
>>
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