Re: [PATCH 9/9] x86/kexec: Use typedef for relocate_kernel_fn function prototype

[David Woodhouse]

On 17 December 2024 10:54:19 CET, Ard Biesheuvel <ardb at kernel.org> wrote:
>On Tue, 17 Dec 2024 at 10:42, David Woodhouse <dwmw2 at infradead.org> wrote:
>> Hm, I am perfectly happy to believe that my memory is failing me, especially when it comes to specifics of i386 assembler code. But are you also telling me that
>> <https://kernelnewbies.org/FAQ/asmlinkage> is a lie?
>>
>
>It seems wildly out of date, at least.
>
>Commit 96a388de5dc53a8b2 from 2007 removed the asmlinkage definition
>containing regparm(0) from include/asm-i386/linkage.h, and I'm not
>convinced it was ever sound to conflate linkage with calling
>convention like that. Today, asmlinkage evaluates to 'extern "C"' when
>using a C++ compiler, which is also not part of the type.
>
>However, I failed to notice that this just moves code around, and only
>applies to 32-bit in the first place. So I won't waste any more of
>your time obsessing over this.

Too late :)

You've already made me concerned about what the calling convention *is* for relocate_kernel() on i386. Because if asmlinkage doesn't mean regparm(0) any more and the i386 kernel is still built with -mregparm=3, then how does the asm code (which seems to believe all its arguments are on the stack) actually work?

It seems slightly unlikely that kexec on i386 has just been broken since 2007 but I'm not sure I'd completely rule it out.

So now I guess I have to actually build a 32-bit userspace test case and *test* it.

And that means I no longer have any excuse for not doing all the same cleanups in the i386 version of the code that I've done for x86_64...

Thanks for that :)