Re: kern/efi/mm.c - MAX_USABLE_ADDRESS

[Leif Lindholm]

On Mon, Dec 09, 2013 at 08:43:57PM +0100, Vladimir 'φ-coder/phcoder' Serbinenko 
wrote:
> If you want to increase it to 0xffffffffffffffff you'll need patch at
> bottom of this mail. All other uses of MAX_USABLE_ADDRESS seem to be
> fine.

I can limit myself to 48 bits addressing due to current architectural
restrictions, so not strictly speaking required, but...

> With this patch we would lose the last usable page but it's just
> 4K and this page is dangerous for overflows anyway so better to avoid.
> I'd suggest using something lower (perhaps 1M lower) to avoid potential
> bugs in EFI.

...this certainly looks like a good idea regardless.

> diff --git a/grub-core/kern/efi/mm.c b/grub-core/kern/efi/mm.c
> index 6e9dace..2becb7b 100644
> --- a/grub-core/kern/efi/mm.c
> +++ b/grub-core/kern/efi/mm.c
> @@ -30,6 +30,7 @@
>    ((grub_efi_memory_descriptor_t *) ((char *) (desc) + (size)))
> 
>  #define BYTES_TO_PAGES(bytes)  (((bytes) + 0xfff) >> 12)
> +#define BYTES_TO_PAGES_DOWN(bytes)     ((bytes) >> 12)
>  #define PAGES_TO_BYTES(pages)  ((pages) << 12)
> 
>  #if defined (__code_model_large__) || !defined (__x86_64__)
> @@ -343,9 +344,9 @@ filter_memory_map (grub_efi_memory_descriptor_t
> *memory_map,
>  #if 1
>           if (BYTES_TO_PAGES (filtered_desc->physical_start)
>               + filtered_desc->num_pages
> -             > BYTES_TO_PAGES (MAX_USABLE_ADDRESS+1LL))
> +             > BYTES_TO_PAGES_DOWN (MAX_USABLE_ADDRESS))
>             filtered_desc->num_pages
> -             = (BYTES_TO_PAGES (MAX_USABLE_ADDRESS+1LL)
> +             = (BYTES_TO_PAGES_DOWN (MAX_USABLE_ADDRESS)
>                  - BYTES_TO_PAGES (filtered_desc->physical_start));
>  #endif