Re: [PATCH] Avoid additional relocations in trampoline code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.08.2024 17:29, Frediano Ziglio wrote:
> The trampoline could have "manual" relocation entries (created
> by assembly macros and some code to use them) and (in case of PE)
> normal executable relocations.
> Remove all normal executable ones. In this way we don't have to
> worry about applying both correctly (they need proper order
> which is hard to spot looking at the code).
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>

I think this wants splitting into one patch replacing sym_offs() and a
2nd one introducing the hand-crafted __XEN_VIRT_START additions (which
may want macro-izing). Plus the justification for the change wants
extending, to actually explain what the problem is - after all there's
no issue anywhere right now.

> --- a/xen/arch/x86/boot/trampoline.S
> +++ b/xen/arch/x86/boot/trampoline.S
> @@ -73,7 +73,7 @@ trampoline_protmode_entry:
>          mov     %ecx,%cr4
>  
>          /* Load pagetable base register. */
> -        mov     $sym_offs(idle_pg_table),%eax
> +        mov     $idle_pg_table_pa, %eax
>          add     bootsym_rel(trampoline_xen_phys_start,4,%eax)
>          mov     %eax,%cr3
>  
> @@ -113,7 +113,7 @@ trampoline_protmode_entry:
>          .code64
>  start64:
>          /* Jump to high mappings. */
> -        movabs  $__high_start, %rdi
> +        movabs  $__high_start_pa + __XEN_VIRT_START, %rdi
>          jmpq    *%rdi
>  
>  #include "video.h"
> --- a/xen/arch/x86/boot/wakeup.S
> +++ b/xen/arch/x86/boot/wakeup.S
> @@ -99,7 +99,7 @@ wakeup_32:
>          mov     $bootsym_rel(wakeup_stack, 4, %esp)
>  
>          # check saved magic again
> -        mov     $sym_offs(saved_magic),%eax
> +        mov     $saved_magic_pa, %eax
>          add     bootsym_rel(trampoline_xen_phys_start, 4, %eax)
>          mov     (%eax), %eax
>          cmp     $0x9abcdef0, %eax
> @@ -112,7 +112,7 @@ wakeup_32:
>          mov     %ecx, %cr4
>  
>          /* Load pagetable base register */
> -        mov     $sym_offs(idle_pg_table),%eax
> +        mov     $idle_pg_table_pa ,%eax
>          add     bootsym_rel(trampoline_xen_phys_start,4,%eax)
>          mov     %eax,%cr3
>  
> @@ -156,7 +156,7 @@ wakeup_32:
>          .code64
>  wakeup_64:
>          /* Jump to high mappings and the higher-level wakeup code. */
> -        movabs  $s3_resume, %rbx
> +        movabs  $s3_resume_pa + __XEN_VIRT_START, %rbx
>          jmp     *%rbx
>  
>  bogus_saved_magic:

With the sym_offs() uses gone, I think it would be best if the macro was
#undef-ed ahead of the inclusion of trampoline.S. Since x86_64.S uses the
macro, that'll require careful re-arrangement of #include order.

> --- a/xen/arch/x86/xen.lds.S
> +++ b/xen/arch/x86/xen.lds.S
> @@ -71,7 +71,12 @@ SECTIONS
>    __2M_text_start = .;         /* Start of 2M superpages, mapped RX. */
>  #endif
>  
> -  start_pa = ABSOLUTE(start - __XEN_VIRT_START);
> +#define DEFINE_PA_ADDRESS(sym) sym ## _pa = ABSOLUTE(sym - __XEN_VIRT_START)
> +  DEFINE_PA_ADDRESS(start);
> +  DEFINE_PA_ADDRESS(saved_magic);
> +  DEFINE_PA_ADDRESS(idle_pg_table);
> +  DEFINE_PA_ADDRESS(__high_start);
> +  DEFINE_PA_ADDRESS(s3_resume);
>  
>    . = __XEN_VIRT_START + XEN_IMG_OFFSET;
>    _start = .;

For the cases where in assembly code you add __XEN_VIRT_START this is pretty
odd: You subtract the value here just to add it back there. Did you consider
a more straightforward approach, like introducing absolute xxx_va symbols?

Also, as a general request: Can you please become used to adding meaningful
subject prefixes to your patches?

Jan