[coreboot] Multiboot

Mathias Krause Mathias.Krause at secunet.com
Tue Sep 23 10:10:31 CEST 2008


Robert Millan wrote:
>   - Add "edx" to clobber list.  Together with eax and ecx which are already
>     in input list, this makes the call able to return for any payload that
>     complies with "cdecl" calling convention.


> +
> +static int run_address_multiboot(void *f)
> +{
> +	int ret;
> +	__asm__ __volatile__ ("call *%3" : "=a" (ret) : "a" (MB_MAGIC2), "b" (0xf0000), "c" (f) : "edx");
> +	return ret;
> +}
> +

this assembler inline is not quite correct to be able to let the call
return because the compiler assumes that after this assembler statement
%ecx still holds the value f which hasn't to be the case. It assumes the
same for %ebx but this should be no problem since the calling function
must preserve it's value if it follows the C calling convention. But
%ecx isn't one of the registers that needs to be preserved, so the
assumption the compiler makes here is wrong.

How about this one:

| static int run_address_multiboot(void *f)
| {
|     int ret, dummy;
|     __asm__ __volatile__ ("call *%4" : "=a" (ret), "=c" (dummy) : "a"
(MB_MAGIC2), "b" (0xf0000), "c" (f) : "edx", "memory");
|     return ret;
| }

The memory clobber is needed since you cannot know what the called
function will actually do with the memory and to ensure all cached
values are actually written back to memory before calling f().


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