Re: [PATCH 1/2] hw/arm/raspi4b: fix guest never seeing more than ~1 GiB of RAM

Hi Marcelo,

On 26/7/26 16:41, Marcelo Manzo wrote:
> raspi4_modify_dtb() decides whether to add a second memory node above
> the 1 GiB peripheral hole by checking info->ram_size -- but that field
> is the boot loader's RAM budget for loading the kernel/initrd/dtb
> image, itself always capped to at most UPPER_RAM_BASE - vcram_size by
> raspi_base_machine_init(). Since that capped value can never exceed
> UPPER_RAM_BASE by construction, the condition was never true for any
> raspi4b configuration, and the second node was never added: the guest
> never saw more than ~1 GiB of its nominal RAM, regardless of the
> machine's actual size.
> 
> board_ram_size(info->board_id), computed one line above in the same
> function, is the value that was actually needed -- the board's real
> total RAM, not the boot loader's own budget for where it's allowed to
> place the kernel image.
> 
> Confirmed via direct measurement inside the guest ("free -h" /
> /proc/meminfo) on raspi4b's default 2 GiB configuration, before and
> after:
> 
>      before: MemTotal:  943524 kB (~921 MiB)
>      after:  MemTotal: 1905824 kB (~1861 MiB)
> 
> Also verified against two real, unmodified Raspberry Pi OS releases
> (Debian 11/Bullseye and Debian 13/Trixie): both now report ~1.8 GiB of
> usable RAM instead of ~900 MiB, with clean boots, working SSH, and no
> kernel errors on either.
> 
> Signed-off-by: Marcelo Manzo <[email protected]>
> ---
>   hw/arm/raspi4b.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/arm/raspi4b.c b/hw/arm/raspi4b.c
> index b92840e1b6..ca246ebb35 100644
> --- a/hw/arm/raspi4b.c
> +++ b/hw/arm/raspi4b.c
> @@ -64,7 +64,19 @@ static void raspi4_modify_dtb(const struct arm_boot_info *info, void *fdt)
>   
>       ram_size = board_ram_size(info->board_id);
>   
> -    if (info->ram_size > UPPER_RAM_BASE) {
> +    /*
> +     * Bug: this used to compare info->ram_size (the boot-loader's RAM
> +     * budget for loading the kernel/initrd/dtb, itself capped to at most
> +     * UPPER_RAM_BASE - vcram_size by raspi_base_machine_init()) rather
> +     * than the board's actual total RAM computed just above. Since that
> +     * capped value can never exceed UPPER_RAM_BASE by construction, this
> +     * condition was never true for any raspi4b configuration -- the
> +     * second memory node was never added, and the guest never saw more
> +     * than ~1 GiB regardless of the machine's nominal RAM size. Confirmed
> +     * via direct measurement: default -m 2G returns ~916 MiB from
> +     * "free -h" inside the guest, not 2 GiB.
> +     */

No need to add this comment in the code, we already have the git
history. With the comment removed:
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>

> +    if (ram_size > UPPER_RAM_BASE) {
>           raspi_add_memory_node(fdt, UPPER_RAM_BASE, ram_size - UPPER_RAM_BASE);
>       }
>   }