Re: [PATCH 02/12] RISC-V: MAXPHYSMEM_2GB doesn't depend on CMODEL_MEDLOW

[Geert Uytterhoeven]

Hi Palmer,

On Fri, Nov 19, 2021 at 5:47 PM Palmer Dabbelt <palmer@xxxxxxxxxxxx> wrote:
> From: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
>
> For non-relocatable kernels we need to be able to link the kernel at
> approximately PAGE_OFFSET, thus requiring medany (as medlow requires the
> code to be linked within 2GiB of 0).  The inverse doesn't apply, though:
> since medany code can be linked anywhere it's fine to link it close to
> 0, so we can support the smaller memory config.
>
> Fixes: de5f4b8f634b ("RISC-V: Define MAXPHYSMEM_1GB only for RV32")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>

Thanks for your patch, which is now commit 9f36b96bc70f9707 ("RISC-V:
MAXPHYSMEM_2GB doesn't depend on CMODEL_MEDLOW").

> I found this when going through the savedefconfig diffs for the K210
> defconfigs.  I'm not entirely sure they're doing the right thing here
> (they should probably be setting CMODEL_LOW to take advantage of the
> better code generation), but I don't have any way to test those
> platforms so I don't want to change too much.

I can confirm MAXPHYSMEM_2GB works on K210 with CMODEL_MEDANY.

As the Icicle has 1760 MiB of RAM, I gave it a try with MAXPHYSMEM_2GB
(and CMODEL_MEDANY), too.  Unfortunately it crashes very early
(needs earlycon to see):

    OF: fdt: Ignoring memory range 0x80000000 - 0x80200000
    Machine model: Microchip PolarFire-SoC Icicle Kit
    printk: debug: ignoring loglevel setting.
    earlycon: ns16550a0 at MMIO32 0x0000000020100000 (options '115200n8')
    printk: bootconsole [ns16550a0] enabled
    printk: debug: skip boot console de-registration.
    efi: UEFI not found.
    Unable to handle kernel paging request at virtual address ffffffff87e00001
    Oops [#1]
    Modules linked in:
    CPU: 0 PID: 0 Comm: swapper Not tainted 5.16.0-08771-g85515233477d #56
    Hardware name: Microchip PolarFire-SoC Icicle Kit (DT)
    epc : fdt_check_header+0x14/0x208
     ra : early_init_dt_verify+0x16/0x94
    epc : ffffffff802ddacc ra : ffffffff8082415a sp : ffffffff81203ee0
     gp : ffffffff812ec3a8 tp : ffffffff8120cd80 t0 : 0000000000000005
     t1 : 0000001040000000 t2 : ffffffff80000000 s0 : ffffffff81203f00
     s1 : ffffffff87e00000 a0 : ffffffff87e00000 a1 : 000000040ffffce7
     a2 : 00000000000000e7 a3 : ffffffff8080394c a4 : 0000000000000000
     a5 : 0000000000000000 a6 : 0000000000000000 a7 : 0000000000000000
     s2 : ffffffff81203f98 s3 : 8000000a00006800 s4 : fffffffffffffff3
     s5 : 0000000000000000 s6 : 0000000000000001 s7 : 0000000000000000
     s8 : 0000000020236c20 s9 : 0000000000000000 s10: 0000000000000000
     s11: 0000000000000000 t3 : 0000000000000018 t4 : 00ff000000000000
     t5 : 0000000000000000 t6 : 0000000000000010
    status: 0000000200000100 badaddr: ffffffff87e00001 cause: 000000000000000d
    [<ffffffff802ddacc>] fdt_check_header+0x14/0x208
    [<ffffffff8082415a>] early_init_dt_verify+0x16/0x94
    [<ffffffff80802dee>] setup_arch+0xec/0x4ec
    [<ffffffff80800700>] start_kernel+0x88/0x6d6
    random: get_random_bytes called from
print_oops_end_marker+0x22/0x44 with crng_init=0
    ---[ end trace 903df1a0ade0b876 ]---
    Kernel panic - not syncing: Attempted to kill the idle task!
    ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

So the FDT is at 0xffffffff87e00000, i.e. at 0x7e00000 from the start
of virtual memory (CONFIG_PAGE_OFFSET=0xffffffff80000000), and thus
within the 2 GiB range.

> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -280,7 +280,7 @@ choice
>                 depends on 32BIT
>                 bool "1GiB"
>         config MAXPHYSMEM_2GB
> -               depends on 64BIT && CMODEL_MEDLOW
> +               depends on 64BIT
>                 bool "2GiB"
>         config MAXPHYSMEM_128GB
>                 depends on 64BIT && CMODEL_MEDANY

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds