Re: [PATCH] xen/arm/efi: merge neighboring banks

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 21/03/2025 21:14, Stefano Stabellini wrote:
> When booting from U-Boot bootefi, there can be a high number of
> neighboring RAM banks. See for example:
>
> (XEN) RAM: 0000000000000000 - 0000000000bfffff
> (XEN) RAM: 0000000000c00000 - 0000000000c00fff
> (XEN) RAM: 0000000000c01000 - 0000000000dfffff
> (XEN) RAM: 0000000000e00000 - 000000000279dfff
> (XEN) RAM: 000000000279e000 - 00000000029fffff
> (XEN) RAM: 0000000002a00000 - 0000000008379fff
> (XEN) RAM: 000000000837a000 - 00000000083fffff
> (XEN) RAM: 0000000008400000 - 0000000008518fff
> (XEN) RAM: 0000000008519000 - 00000000085fffff
> (XEN) RAM: 0000000008600000 - 0000000008613fff
> (XEN) RAM: 0000000008614000 - 00000000097fffff
> (XEN) RAM: 0000000009800000 - 00000000098a7fff
> (XEN) RAM: 00000000098a8000 - 0000000009dfffff
> (XEN) RAM: 0000000009e00000 - 0000000009ea7fff
> (XEN) RAM: 0000000009ea8000 - 000000001fffffff
> (XEN) RAM: 0000000020000000 - 000000002007ffff
> (XEN) RAM: 0000000020080000 - 0000000077b17fff
> (XEN) RAM: 0000000077b19000 - 0000000077b2bfff
> (XEN) RAM: 0000000077b2c000 - 0000000077c8dfff
> (XEN) RAM: 0000000077c8e000 - 0000000077c91fff
> (XEN) RAM: 0000000077ca7000 - 0000000077caafff
> (XEN) RAM: 0000000077cac000 - 0000000077caefff
> (XEN) RAM: 0000000077cd0000 - 0000000077cd2fff
> (XEN) RAM: 0000000077cd4000 - 0000000077cd7fff
> (XEN) RAM: 0000000077cd8000 - 000000007bd07fff
> (XEN) RAM: 000000007bd09000 - 000000007fd5ffff
> (XEN) RAM: 000000007fd70000 - 000000007fefffff
> (XEN) RAM: 0000000800000000 - 000000087fffffff
>
> It is undesirable to keep them separate, as this approach consumes more
> resources.
What resources? This is pre-allocated static array in the binary. They 
are also dropped after init. The more interesting argument is...
> Additionally, Xen does not currently support boot modules that span
> multiple banks: at least one of the regions get freed twice. The first
> time from setup_mm->populate_boot_allocator, then again from
> discard_initial_modules->fw_unreserved_regions. With a high number of
> banks, it can be difficult to arrange the boot modules in a way that
> avoids spanning across multiple banks.
... this one. Although, I find weird that U-boot would create tons of 
banks. Have you considered to ask them what's going on?
Also, what about the cases where U-boot is not booting Xen without UEFI? 
Why is this not a problem? Asking just in case the merge should happen 
in code common rather than in UEFI.
> This small patch merges neighboring regions, to make dealing with them
> more efficient, and to make it easier to load boot modules.
While I understand the value for this, I ...

> The patch
> also takes the opportunity to check and discard duplicates.
don't understand why we need to check for duplicates. This also doesn't 
properly work for a few reasons:
  * This doesn't cover partial overlap
  * This would not work if an entry was merged with another previously

So I think the code to check duplicates should be removed. If you are 
concerned about overlap, then it would be better to enable 
check_reserved  just drop the code. If you are concerned about to detect 
and warn/panic.

> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
>
> diff --git a/xen/arch/arm/efi/efi-boot.h b/xen/arch/arm/efi/efi-boot.h
> index a80a5a7ab3..f6f23ed5b2 100644
> --- a/xen/arch/arm/efi/efi-boot.h
> +++ b/xen/arch/arm/efi/efi-boot.h
> @@ -163,6 +163,20 @@ static bool __init meminfo_add_bank(struct membanks *mem,
>       struct membank *bank;
>       paddr_t start = desc->PhysicalStart;
>       paddr_t size = desc->NumberOfPages * EFI_PAGE_SIZE;
> +    int j;
nr_banks is an "unsigned int". So this should be the same type.

> +
> +    for ( j = 0; j < mem->nr_banks; j++ )
> +    {
> +        if ( mem->bank[j].start == start && mem->bank[j].size == size )
> +        {
> +            return true;
> +        }
> +        else if ( mem->bank[j].start + mem->bank[j].size == start )
Please add some parentheses to make it more obvious that you checking (a 
+ b) == size.
> +        {
> +            mem->bank[j].size += size;
> +            return true;
> +        }
> +    }
>   
>       if ( mem->nr_banks >= mem->max_banks )
>           return false;
Cheers,

--
Julien Grall