linux-kernel - Re: Non-booting current Linus' tree

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Message-ID: <CA+55aFyOMm4Tu3RPao4fQ=Ek0KH=8LSn1hoFRXp-L+Js3GeMvQ@mail.gmail.com>
Date:	Fri, 3 Jul 2015 14:40:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: Non-booting current Linus' tree

On Fri, Jul 3, 2015 at 8:23 AM, Jan Kara <jack@...e.cz> wrote:
>
> Because the address isn't 32-byte aligned (which I assume is the
> requirement from looking into the code). So clearly my gcc messed up and
> miscompiled the thing by ignoring the alignment attribute.

Well, it's probably a mistake to begin with to expect gcc to get stack
alignment right. Especially since we tell gcc to not align the stack
as much as it usually wants to with -mpreferred-stack-boundary=2.

The code is broken in other ways too. The fxsave alignment isn't 32
bytes. It's 16 bytes. And that's already encoded in the "struct
fxregs_state", so adding that extra alignment is just bogus crud
anyway.

So I seriously think that the whole commit 91a8c2a5b43f is just
fundamentally broken and should probably be reverted. The theoretical
issue it fixes is a smaller problem than the broken code it introduces
(and I'm not just talking about gcc bugs).

Plus the code just sets up and writes to a global variable *anyway*,
so the alleged race with using a static allocation is bogus: if that's
a real concern, then the code is fundamentally buggy in other ways
anyway.

I don't think you can boot with different CPU's having different mxcsr
features anyway, so..

Ingo?

               Linus
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