Re: [Xen-devel] [PATCH v2 06/23] x86/boot: use %ecx instead of %eax

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Jul 20, 2015 at 04:29:01PM +0200, Daniel Kiper wrote:
> Use %ecx instead of %eax to store low memory upper limit from EBDA.
> This way we do not wipe multiboot protocol identifier. It is needed
> in reloc() to differentiate between multiboot (v1) and
> multiboot2 protocol.
> 
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>  xen/arch/x86/boot/head.S |   24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
> index 3cbb2e6..77e7da9 100644
> --- a/xen/arch/x86/boot/head.S
> +++ b/xen/arch/x86/boot/head.S
> @@ -87,14 +87,14 @@ __start:
>          jne     not_multiboot
>  
>          /* Set up trampoline segment 64k below EBDA */
> -        movzwl  0x40e,%eax          /* EBDA segment */
> -        cmp     $0xa000,%eax        /* sanity check (high) */
> +        movzwl  0x40e,%ecx          /* EBDA segment */
> +        cmp     $0xa000,%ecx        /* sanity check (high) */
>          jae     0f
> -        cmp     $0x4000,%eax        /* sanity check (low) */
> +        cmp     $0x4000,%ecx        /* sanity check (low) */
>          jae     1f
>  0:
> -        movzwl  0x413,%eax          /* use base memory size on failure */
> -        shl     $10-4,%eax
> +        movzwl  0x413,%ecx          /* use base memory size on failure */
> +        shl     $10-4,%ecx
>  1:
>          /*
>           * Compare the value in the BDA with the information from the
> @@ -106,21 +106,21 @@ __start:
>          cmp     $0x100,%edx         /* is the multiboot value too small? */
>          jb      2f                  /* if so, do not use it */
>          shl     $10-4,%edx
> -        cmp     %eax,%edx           /* compare with BDA value */
> -        cmovb   %edx,%eax           /* and use the smaller */
> +        cmp     %ecx,%edx           /* compare with BDA value */
> +        cmovb   %edx,%ecx           /* and use the smaller */
>  
>  2:      /* Reserve 64kb for the trampoline */
> -        sub     $0x1000,%eax
> +        sub     $0x1000,%ecx
>  
>          /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
> -        xor     %al, %al
> -        shl     $4, %eax
> -        mov     %eax,sym_phys(trampoline_phys)
> +        xor     %cl, %cl
> +        shl     $4, %ecx
> +        mov     %ecx,sym_phys(trampoline_phys)
>  
>          /* Save the Multiboot info struct (after relocation) for later use. 
> */
>          mov     $sym_phys(cpu0_stack)+1024,%esp
>          push    %ebx                /* Multiboot information address. */
> -        push    %eax                /* Boot trampoline address. */
> +        push    %ecx                /* Boot trampoline address. */
>          call    reloc
>          add     $8,%esp             /* Remove reloc() args from stack. */
>          mov     %eax,sym_phys(multiboot_ptr)
> -- 
> 1.7.10.4
> 
> 
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