Re: [PATCH v1 1/1] x86/Documentation: Update algo in init_size description of boot protocol

[Randy Dunlap]

Hi Andy,

On 11/25/24 12:31 AM, Andy Shevchenko wrote:
> The init_size description of boot protocol has an example of the runtime
> start address for the compressed bzImage. For non-relocatable kernel
> it relies on the pref_address value (if not 0), but for relocatable case
> only pays respect to the load_addres and kernel_alignment, and it is
> inaccurate for the latter. Boot loader must consider the pref_address
> as the Linux kernel relocates to it before being decompressed as nicely
> described in the commit 43b1d3e68ee7 message.
> 
> Due to this inaccuracy some of the bootloaders (*) made a mistake in
> the calculations and if kernel image is big enough, this may lead to
> unbootable configurations.
> 
> *)
>   In particular, kexec-tools missed that and resently got a couple of
>   changes which will be part of v2.0.30 release. For the record,
>   the 43b1d3e68ee7 fixed only kernel kexec implementation and also missed
>   to update the init_size description.
> 
> While at it, make an example C-like looking as it's done elsewhere in
> the document and fix indentation, so the syntax highliting will work
> properly in some editors (vim).
> 
> Fixes: 43b1d3e68ee7 ("kexec: Allocate kernel above bzImage's pref_address")
> Fixes: d297366ba692 ("x86: document new bzImage fields")
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
>  Documentation/arch/x86/boot.rst | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/arch/x86/boot.rst b/Documentation/arch/x86/boot.rst
> index 4fd492cb4970..01f08d94e8df 100644
> --- a/Documentation/arch/x86/boot.rst
> +++ b/Documentation/arch/x86/boot.rst
> @@ -896,10 +896,19 @@ Offset/size:	0x260/4
>  
>    The kernel runtime start address is determined by the following algorithm::
>  
> -	if (relocatable_kernel)
> -	runtime_start = align_up(load_address, kernel_alignment)
> -	else
> -	runtime_start = pref_address
> +    if ( relocatable_kernel ) {
> +      if ( load_address < pref_address )

What's up with the extra spaces around ( and ) ... and inconsistent with
the lines below?

> +        load_address = pref_address;
> +      runtime_start = align_up(load_address, kernel_alignment);
> +    } else {
> +      runtime_start = pref_address;
> +    }
> +
> +Hence the necessary memory window location and size can be estimated by
> +a boot loader as::
> +
> +    memory_window_start = runtime_start;
> +    memory_window_size = init_size;
>  
>  ============	===============
>  Field name:	handover_offset

-- 
~Randy