Re: [PATCH v2 3/5] x86: Force proper gdt_boot_base setting

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.08.2024 10:34, Frediano Ziglio wrote:
> Instead of relocate the value at that position compute it
> entirely and write it.
> During EFI boots sym_offs(SYMBOL) are potentially relocated
> causing the values to be corrupted.

This requires quite a bit more explanation, also to understand why a
lone specific sym_offs() is being dealt with here, leaving others
untouched. I'm specifically puzzled by the two in the MB2 header: If
the GDT one is a problem, why would those not be? Of course similarly
for others, in particular those used to pre-fill page tables. I think
an adjustment here needs to come with the addition of a comment next
to the #define, to clarify where the use is appropriate and where it
needs to be avoided.

> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -132,8 +132,7 @@ multiboot2_header:
>  gdt_boot_descr:
>          .word   .Ltrampoline_gdt_end - trampoline_gdt - 1
>  gdt_boot_base:
> -        .long   sym_offs(trampoline_gdt)
> -        .long   0 /* Needed for 64-bit lgdt */
> +        .quad   0 /* Needed for 64-bit lgdt */

The comment is now somewhat odd: It's no longer the entire line that's
there just because we want to use LGDT from 64-bit code, but just half
of what the .quad produces. Therefore perhaps either keep as two longs
(maybe with a different brief comment put on the former), or adjust
the comment wording.

> @@ -373,15 +372,16 @@ __efi64_mb2_start:
>  x86_32_switch:
>          mov     %r15,%rdi
>  
> -        /* Store Xen image load base address in place accessible for 32-bit 
> code. */
> -        lea     __image_base__(%rip),%esi
> -
>          cli
>  
>          /* Initialize GDTR. */
> -        add     %esi,gdt_boot_base(%rip)
> +        lea     trampoline_gdt(%rip), %esi
> +        mov     %esi, gdt_boot_base(%rip)
>          lgdt    gdt_boot_descr(%rip)
>  
> +        /* Store Xen image load base address in place accessible for 32-bit 
> code. */
> +        lea     __image_base__(%rip),%esi

What's the point in moving this code? Surely you could use another
register for the LEA/MOV pair above? In any event - _if_ you move
the code, please also add the blank missing after the comma.

> @@ -439,7 +439,8 @@ __pvh_start:
>          movb    $-1, sym_esi(opt_console_xen)
>  
>          /* Prepare gdt and segments */
> -        add     %esi, sym_esi(gdt_boot_base)
> +        lea     sym_esi(trampoline_gdt), %ecx
> +        mov     %ecx, sym_esi(gdt_boot_base)
>          lgdt    sym_esi(gdt_boot_descr)
>  
>          mov     $BOOT_DS, %ecx
> @@ -543,7 +544,8 @@ trampoline_bios_setup:
>           *
>           * Initialize GDTR and basic data segments.
>           */
> -        add     %esi,sym_esi(gdt_boot_base)
> +        lea     sym_esi(trampoline_gdt), %ecx
> +        mov     %ecx, sym_esi(gdt_boot_base)
>          lgdt    sym_esi(gdt_boot_descr)
>  
>          mov     $BOOT_DS,%ecx

While you mention that you make these changes for consistency, I'm
afraid I don't really see the point. The paths are all different
anyway - there's nothing wrong with leaving everything except
x86_32_switch untouched. Far less code churn then. All it would take
is extending the comment there a little to mention why the value is
fully overwritten rather than merely adjusted.

Jan