linux-kernel - Re: [PATCH] Make alignment cflags configurable.

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Message-ID: <3447724.z50aeuJHce@aragorn.auenland.lan>
Date:	Thu, 13 Aug 2015 00:30:19 +0200
From:	Jan-Simon Moeller <dl9pf@....de>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	llvmlinux@...ts.linuxfoundation.org, behanw@...verseincode.com
Subject: Re: [PATCH] Make alignment cflags configurable.

Hi all!

> You could mention that this is to fix the clang build. But why is it
> needed? It isn't that clang just doesn't accept the option, is it?
> Otherwise we could just use $(call cc-option, -falign-jumps=1) etc.

Yes it is to fix the build with clang. 
I tried cc-option, but it does not improve the situation (more below).
This is  why I chose the config option approach in the patch.

> Did you get to the bottom of the clang failure here? Just turning this
> off without a coherent explanation doesn't seem like the right thing to
> do.

I know it is not the final solution which is why I turned it into a config 
option. We can still debate if default should be "y" or "n". This way we all 
can proceed.

@Ingo: would it be fine if we wrap it into a config option defaulting to "y" ?

What I can say so far is that although clang warns about the unknown option
and ignores it, the resulting kernel still fails to boot somewhere early in 
start_kernel(). I'm still investigating. 

My current trace ends like this:
page_address_init ~ setup_arch ~ then arch/x86/kernel/setup.c:898
setup.c:898 is a printk actually ... 
early_idt_handler_array[i]  ~> early_idt_handler_common

The mail thread is here:
http://lists.linuxfoundation.org/pipermail/llvmlinux/2015-August/001276.html

<wild guess>
We still build with -no-integrated-as which means we use gas. Maybe the flag
is passed-on there and things get confused. 
</wile guess>

Best,
Jan-Simon
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