Re: [PATCH v2] hw: riscv: Allow large kernels to boot by moving the initrd further away in RAM

[Alistair Francis]

On Wed, Feb 7, 2024 at 1:42 AM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
>
> Currently, the initrd is placed at 128MB, which overlaps with the kernel
> when it is large (for example syzbot kernels are). From the kernel side,
> there is no reason we could not push the initrd further away in memory
> to accommodate large kernels, so move the initrd at 512MB when possible.
>
> The ideal solution would have been to place the initrd based on the
> kernel size but we actually can't since the bss size is not known when
> the image is loaded by load_image_targphys_as() and the initrd would
> then overlap with this section.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>
> Changes in v2:
> - Fix typos in commit log (Daniel) and title
> - Added to the commit log why using the kernel size does not work
>   (Daniel)
>
>  hw/riscv/boot.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/hw/riscv/boot.c b/hw/riscv/boot.c
> index 0ffca05189..9a367af2fa 100644
> --- a/hw/riscv/boot.c
> +++ b/hw/riscv/boot.c
> @@ -188,13 +188,13 @@ static void riscv_load_initrd(MachineState *machine, 
> uint64_t kernel_entry)
>       * kernel is uncompressed it will not clobber the initrd. However
>       * on boards without much RAM we must ensure that we still leave
>       * enough room for a decent sized initrd, and on boards with large
> -     * amounts of RAM we must avoid the initrd being so far up in RAM
> -     * that it is outside lowmem and inaccessible to the kernel.
> -     * So for boards with less  than 256MB of RAM we put the initrd
> -     * halfway into RAM, and for boards with 256MB of RAM or more we put
> -     * the initrd at 128MB.
> +     * amounts of RAM, we put the initrd at 512MB to allow large kernels
> +     * to boot.
> +     * So for boards with less than 1GB of RAM we put the initrd
> +     * halfway into RAM, and for boards with 1GB of RAM or more we put
> +     * the initrd at 512MB.
>       */
> -    start = kernel_entry + MIN(mem_size / 2, 128 * MiB);
> +    start = kernel_entry + MIN(mem_size / 2, 512 * MiB);
>
>      size = load_ramdisk(filename, start, mem_size - start);
>      if (size == -1) {
> --
> 2.39.2
>
>