[RFC PATCH 1/3] riscv: Remove 2MB offset in the mm layout

Wed Nov 24 07:09:16 PST 2021

On 11/24/21 12:58, Guo Ren wrote:
> On Tue, Nov 23, 2021 at 9:37 PM Alexandre Ghiti
> <alexandre.ghiti at canonical.com> wrote:
>> On Tue, Nov 23, 2021 at 4:56 AM Anup Patel <anup at brainfault.org> wrote:
>>> +Alex
>>>
>>> On Tue, Nov 23, 2021 at 7:27 AM <guoren at kernel.org> wrote:
>>>> From: Guo Ren <guoren at linux.alibaba.com>
>>>>
>>>> The current RISC-V's mm layout is based on a 2MB offset and wasting
>>>> memory. Remove 2MB offset and map PAGE_OFFSET at start_of_DRAM.
>>>> Then we could reduce the memory reserved for opensbi in the next
>>>> patch.
>>> The real problem is that the generic kernel marks memory before
>>> __pa(PAGE_OFFSET) as reserved which is evident from the boot
>>> print "OF: fdt: Ignoring memory range 0x80000000 - 0x80200000".
>> Agreed, memblock 'rejects' this region because MIN_MEMBLOCK_ADDR is
>> defined as __pa(PAGE_OFFSET) which points to 0x80200000. I have a
>> patch to enable the use of hugepages for the linear mapping [1] that
>> does just that, things are not that easy since then it breaks initrd
>> initialization which is an early caller of __va, I updated this
>> patchset a few months ago, I can push that soon @Guo Ren.
> Seems your patch makes the mapping of early_pg_dir & swapper_pg_dir different.
>
> early_pg_dir: 0x80200000 -> 0xffffffe000000000
> swapper_pg_dir: 0x80000000 -> 0xffffffe000000000
>
> It breaks the vmlinux.ld.S, doesn't it?

Indeed, early_pg_dir and swapper_pg_dir have different mappings, but 
that's because when establishing the early_pg_dir mapping, the only 
piece of information we have is the load address of the kernel, which is 
0x8020_0000 (or whatever). And that breaks initrd because 
__early_init_dt_declare_initrd calls __va in between and then when 
swapper_pg_dir is used, it breaks because the mappings differ. I did not 
find any better way to do that, and IIRC arm64 has a similar issue.

>
>> [1] https://lkml.org/lkml/2020/6/3/696
>>
>>
>>
>>> One simple way to re-claim the first 2MB of memory is by:
>>> 1) Not placing OpenSBI firmware at start of RAM and rather
>>> place it towards end/middle or RAM away from kernel and initrd
>>> 2) Load kernel at start of the RAM
>>>
>>> The point#1 is already supported by OpenSBI firmwares using
>>> position independent compilation. In fact, U-Boot SPL does
>>> not load OpenSBI firmware at the start of RAM.
>>>
>>> I would suggest Allwinner D1 to follow U-Boot SPL and have
>>> the booting stage before OpenSBI to load OpenSBI firmware
>>> somewhere else.
>>>
>>> Regards,
>>> Anup
>>>
>>>> Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
>>>> Cc: Palmer Dabbelt <palmer at dabbelt.com>
>>>> Cc: Anup Patel <anup.patel at wdc.com>
>>>> Cc: Atish Patra <atishp at rivosinc.com>
>>>> ---
>>>>   arch/riscv/include/asm/page.h   |  8 ++++++++
>>>>   arch/riscv/kernel/head.S        | 10 +++-------
>>>>   arch/riscv/kernel/vmlinux.lds.S |  5 ++---
>>>>   arch/riscv/mm/init.c            | 11 ++++++++---
>>>>   4 files changed, 21 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
>>>> index b3e5ff0125fe..299147c78b4a 100644
>>>> --- a/arch/riscv/include/asm/page.h
>>>> +++ b/arch/riscv/include/asm/page.h
>>>> @@ -16,6 +16,14 @@
>>>>   #define PAGE_SIZE      (_AC(1, UL) << PAGE_SHIFT)
>>>>   #define PAGE_MASK      (~(PAGE_SIZE - 1))
>>>>
>>>> +#if __riscv_xlen == 64
>>>> +/* Image load offset(2MB) from start of RAM */
>>>> +#define LOAD_OFFSET    0x200000
>>>> +#else
>>>> +/* Image load offset(4MB) from start of RAM */
>>>> +#define LOAD_OFFSET    0x400000
>>>> +#endif
>>>> +
>>>>   #ifdef CONFIG_64BIT
>>>>   #define HUGE_MAX_HSTATE                2
>>>>   #else
>>>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>>>> index f52f01ecbeea..a6ac892d2ccf 100644
>>>> --- a/arch/riscv/kernel/head.S
>>>> +++ b/arch/riscv/kernel/head.S
>>>> @@ -61,13 +61,7 @@ ENTRY(_start)
>>>>          /* Image load offset (0MB) from start of RAM for M-mode */
>>>>          .dword 0
>>>>   #else
>>>> -#if __riscv_xlen == 64
>>>> -       /* Image load offset(2MB) from start of RAM */
>>>> -       .dword 0x200000
>>>> -#else
>>>> -       /* Image load offset(4MB) from start of RAM */
>>>> -       .dword 0x400000
>>>> -#endif
>>>> +       .dword LOAD_OFFSET
>>>>   #endif
>>>>          /* Effective size of kernel image */
>>>>          .dword _end - _start
>>>> @@ -94,6 +88,8 @@ relocate:
>>>>          la a1, kernel_map
>>>>          XIP_FIXUP_OFFSET a1
>>>>          REG_L a1, KERNEL_MAP_VIRT_ADDR(a1)
>>>> +       li a2, LOAD_OFFSET
>>>> +       add a1, a1, a2
>>>>          la a2, _start
>>>>          sub a1, a1, a2
>>>>          add ra, ra, a1
>>>> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
>>>> index 5104f3a871e3..75b7c72cd4bd 100644
>>>> --- a/arch/riscv/kernel/vmlinux.lds.S
>>>> +++ b/arch/riscv/kernel/vmlinux.lds.S
>>>> @@ -11,10 +11,9 @@
>>>>   #else
>>>>
>>>>   #include <asm/pgtable.h>
>>>> -#define LOAD_OFFSET KERNEL_LINK_ADDR
>>>>
>>>> -#include <asm/vmlinux.lds.h>
>>>>   #include <asm/page.h>
>>>> +#include <asm/vmlinux.lds.h>
>>>>   #include <asm/cache.h>
>>>>   #include <asm/thread_info.h>
>>>>   #include <asm/set_memory.h>
>>>> @@ -32,7 +31,7 @@ PECOFF_FILE_ALIGNMENT = 0x200;
>>>>   SECTIONS
>>>>   {
>>>>          /* Beginning of code and text segment */
>>>> -       . = LOAD_OFFSET;
>>>> +       . = LOAD_OFFSET + KERNEL_LINK_ADDR;
>>>>          _start = .;
>>>>          HEAD_TEXT_SECTION
>>>>          . = ALIGN(PAGE_SIZE);
>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>> index 24b2b8044602..920e78f8c3e4 100644
>>>> --- a/arch/riscv/mm/init.c
>>>> +++ b/arch/riscv/mm/init.c
>>>> @@ -221,6 +221,11 @@ static void __init setup_bootmem(void)
>>>>          if (!IS_ENABLED(CONFIG_BUILTIN_DTB))
>>>>                  memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
>>>>
>>>> +       /*
>>>> +        * Reserve OpenSBI region and depends on PMP to deny accesses.
>>>> +        */
>>>> +       memblock_reserve(__pa(PAGE_OFFSET), LOAD_OFFSET);
>>>> +
>>>>          early_init_fdt_scan_reserved_mem();
>>>>          dma_contiguous_reserve(dma32_phys_limit);
>>>>          if (IS_ENABLED(CONFIG_64BIT))
>>>> @@ -604,7 +609,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>
>>>>          kernel_map.va_kernel_xip_pa_offset = kernel_map.virt_addr - kernel_map.xiprom;
>>>>   #else
>>>> -       kernel_map.phys_addr = (uintptr_t)(&_start);
>>>> +       kernel_map.phys_addr = (uintptr_t)(&_start) - LOAD_OFFSET;
>>>>          kernel_map.size = (uintptr_t)(&_end) - kernel_map.phys_addr;
>>>>   #endif
>>>>          kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
>>>> @@ -645,8 +650,8 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>          create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>>>>                             kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
>>>>   #else
>>>> -       create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>>>> -                          kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
>>>> +       create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr + LOAD_OFFSET,
>>>> +                          kernel_map.phys_addr + LOAD_OFFSET, PMD_SIZE, PAGE_KERNEL_EXEC);
>>>>   #endif
>>>>   #else
>>>>          /* Setup trampoline PGD */
>>>> --
>>>> 2.25.1
>>>>
>
>