Re: [PATCH 2/7] multiboot2: Allow 64-bit entry tags

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Mar 13, 2024 at 03:07:43PM +0000, Ross Lagerwall wrote:
> Binaries may be built with entry points above 4G. While bootloaders may
> relocate them below 4G, it should be possible for the binary to specify
> those entry points. Therefore, extend the multiboot2 protocol such that
> 64 bit addresses are allowed for entry points. The extension is done in
> a backwards-compatible way.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> ---
>  doc/multiboot.texi | 32 +++++++++++++++++++-------------
>  doc/multiboot2.h   |  6 +++++-
>  2 files changed, 24 insertions(+), 14 deletions(-)
> 
> diff --git a/doc/multiboot.texi b/doc/multiboot.texi
> index d12719c744eb..049afab53c1f 100644
> --- a/doc/multiboot.texi
> +++ b/doc/multiboot.texi
> @@ -522,12 +522,12 @@ header.
>  
>  @example
>  @group
> -        +-------------------+
> -u16     | type = 3          |
> -u16     | flags             |
> -u32     | size              |
> -u32     | entry_addr        |
> -        +-------------------+
> +          +-------------------+
> +u16       | type = 3          |
> +u16       | flags             |
> +u32       | size              |
> +u32 / u64 | entry_addr        |
> +          +-------------------+

I might be confused, but this entry point is used in 32bit protected
mode, and hence a 64bit value is simply impossible to use according to
the protocol in "3.3 I386 machine state".

Unless that section is expanded to describe other protocols that use
the entry address in a way where 64bits could be meaningful it seems
pointless to expand the field.

>  @end group
>  @end example
>  
> @@ -538,7 +538,10 @@ The meaning of each is as follows:
>  
>  @item entry_addr
>  The physical address to which the boot loader should jump in order to
> -start running the operating system.
> +start running the operating system. @samp{entry_addr} may be specified
> +either as a @samp{u32} or @samp{u64}. The bootloader should use the
> +header size to determine the size of @samp{entry_addr}.
> +
>  @end table
>  
>  @subsection EFI i386 entry address tag of Multiboot2 header
> @@ -573,12 +576,12 @@ tag of Multiboot2 header are ignored.
>  
>  @example
>  @group
> -        +-------------------+
> -u16     | type = 9          |
> -u16     | flags             |
> -u32     | size              |
> -u32     | entry_addr        |
> -        +-------------------+
> +          +-------------------+
> +u16       | type = 9          |
> +u16       | flags             |
> +u32       | size              |
> +u32 / u64 | entry_addr        |
> +          +-------------------+

This does seem sensible.

Thanks, Roger.