[PATCH] arm64: booting: Require placement within 48-bit addressable memory

[Mark Rutland]

On Mon, Nov 14, 2022 at 12:58:10PM +0100, Ard Biesheuvel wrote:
> Some configurations (i.e., 64k + LVA/LPA) can tolerate a physical
> placement of the kernel image outside of the 48-bit addressable region,
> but given that the loader has no way of knowing whether or not the image
> in question supports LVA/LPA, it currently has no choice but to place it
> below the 48-bit mark.
> 
> Once we add support for LPA2, which allows 52-bit physical and virtual
> addressing when using 4k or 16k pages, but in way that relies on
> increasing the number of paging levels, there will be more variety in
> the configurations that may or may not support this.
> 
> So repaint bit #3 in the Image header as 'must be placed within 48-bit
> addressable memory', as this is the current de facto meaning, and add
> bits that we may set later to allow placement outside of that.

Could we just clean up the definition of bit 3 for now, then add the other bits
when we actually set them?

> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  Documentation/arm64/booting.rst | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
> index 8c324ad638de2b27..9919e1a107add655 100644
> --- a/Documentation/arm64/booting.rst
> +++ b/Documentation/arm64/booting.rst
> @@ -114,16 +114,21 @@ Header notes:
>  			* 1 - 4K
>  			* 2 - 16K
>  			* 3 - 64K
> -  Bit 3		Kernel physical placement
> +  Bit 3-4	Kernel physical placement
>  
>  			0
>  			  2MB aligned base should be as close as possible
>  			  to the base of DRAM, since memory below it is not
>  			  accessible via the linear mapping
>  			1
> +			  2MB aligned base such that base plus image_size is
> +			  entirely within the 48-bit addressable range

This seems to assume text_offset is zero (which is the case today, but this is
the first part of booting.txt that assumes that); as a matter of policy are we
sure that we'll never use a non-zero offset again? If so, we probably want to
mention that in the "header notes" section, or come up with some wording to
include it here.

Otherwise, this looks good; I'm certainly in favour of explicitly mentioning
the 48-bit restriction.

Thanks,
Mark.

> +			2
> +			  Reserved
> +			3
>  			  2MB aligned base may be anywhere in physical
>  			  memory
> -  Bits 4-63	Reserved.
> +  Bits 5-63	Reserved.
>    ============= ===============================================================
>  
>  - When image_size is zero, a bootloader should attempt to keep as much
> -- 
> 2.35.1
>