[coreboot] Multiboot

Mathias Krause Mathias.Krause at secunet.com
Wed Sep 24 08:48:44 CEST 2008

Robert Millan wrote:
> On Tue, Sep 23, 2008 at 04:11:34PM +0200, Mathias Krause wrote:
>>> I see.  But why not just add "ecx" to clobber list instead?  Then a dummy
>>> variable isn't needed.
>> When doing so the compiler is unable to find another register called
>> %ecx to assign the value of f as input value. He cannot use %ecx because
>> by mentioning it in the clobber list you already told him that register
>> may change it's value at any time within the asm statement. That for the
>> "kudge" with the dummy output variable is needed but it should get
>> optimized away since it's value is never used. It's only usage is to
>> tell the compiler that the value of %ecx has changed.
> Ok.
>>>> 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().
>>> Is this really a problem?  If the payload expects to return, it isn't
>>> supposed to modify coreboot's memory at all.  If it does, I'd say it's
>>> normal that things break.
>> Not the return of f() is the problem but missing memory writes _before_
>> calling it. If coreboot does some memory modifications that are relevant
>> for f() they should be taken out of registers and written back to memory
>> before calling f(). Otherwise f() may break.
> Makes sense.  Thanks for the explanation.

You're welcome.

> Here's a new patch incorporating your suggestions.



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