Re: [PATCH 2/5] x86: Fix early output messages in case of EFI

[Jan Beulich <jbeulich@suse.com>]

On 07.08.2024 15:48, Alejandro Vallejo wrote:
> If code is loaded by EFI the loader will relocate the image
> under 4GB.

This is the MB2 EFI path you're talking about? Since there are two paths,
I think this needs clearly separating in all descriptions.

If it is the MB2 path, then "relocate" isn't quite correct, I think:
Relocations aren't applied in that case, as none are present in xen.gz.
I'd rather call this "put at an address below 4G". However, that isn't
any different from the non-EFI MB1/2 paths, is it? I feel like I'm
missing something here.

> This cause offsets in x86 code generated by
> sym_offs(SYMBOL) to be relocated too (basically they won't be
> offsets from image base). In order to get real offset the
> formulae "sym_offs(SYMBOL) - sym_offs(__image_base__)" is
> used instead.

The main calculations of %esi are, if I'm not mistaken,

        /* Store Xen image load base address in place accessible for 32-bit code. */
        lea     __image_base__(%rip),%esi

and

        /* Calculate the load base address. */
        call    1f
1:      pop     %esi
        sub     $sym_offs(1b), %esi

i.e. both deliberately %rip-relative to be position-independent. What's
wrong with this?

There are many more uses of sym_esi(). Why is it only this single one
which poses a problem?

> Also, in some case %esi register (that should point to
> __image_base__ addresss) is not set so compute in all cases.

Which "some case" is this?

> Code tested forcing failures in the code.
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>

No Fixes: tag?

Jan