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

From: Alexandre Ghiti
Date: Tue Nov 23 2021 - 08:37:35 EST

On Tue, Nov 23, 2021 at 4:56 AM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
>
> +Alex
>
> On Tue, Nov 23, 2021 at 7:27 AM <guoren@xxxxxxxxxx> wrote:
> >
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > 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.

[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@xxxxxxxxxxxxxxxxx>
> > Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> > Cc: Anup Patel <anup.patel@xxxxxxx>
> > Cc: Atish Patra <atishp@xxxxxxxxxxxx>
> > ---
> > 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
> >