Re: [PATCH] x86/kexec: Replace CALL+UD2 pattern with JMP

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.12.2025 16:29, Andrew Cooper wrote:
> Linux jumps to the target image rather than calling it.  Switch to using JMP,
> and drop the trailing UD2s.
> 
> Linux does have a mode, named CONFIG_KEXEC_JUMP, where the target image can be
> returned from, but that involves extra metadata and setting up a stack within
> the target image which Xen doesn't support at the moment.
> 
> No functional change.
> 
> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I wonder though why CALL was used in the first place. Usually this would be
to e.g. have the callee have a call frame in order to e.g. unwind. For kexec
that may not be overly relevant though.

Jan

> --- a/xen/arch/x86/x86_64/kexec_reloc.S
> +++ b/xen/arch/x86/x86_64/kexec_reloc.S
> @@ -75,10 +75,8 @@ FUNC(kexec_reloc, PAGE_SIZE)
>          testq   $KEXEC_RELOC_FLAG_COMPAT, %r8
>          jnz     .L_call_32_bit
>  
> -.L_call_64_bit:
> -        /* Call the image entry point.  This should never return. */
> -        callq   *%rbp
> -        ud2
> +        /* Jump to the image entry point */
> +        jmp     *%rbp
>  
>  .L_call_32_bit:
>          /* Setup IDT. */
> @@ -170,9 +168,8 @@ FUNC_LOCAL(compatibility_mode)
>          xorl    %eax, %eax
>          movl    %eax, %cr4
>  
> -        /* Call the image entry point.  This should never return. */
> -        call    *%ebp
> -        ud2
> +        /* Jump to the image entry point. */
> +        jmp     *%ebp
>  END(compatibility_mode)
>  
>          /* Separate code and data into into different cache lines */
> 
> base-commit: 351d41e8aecc3f7566a0baa7b4066d06dedd7113