Date: Wed, 8 Mar 2017 22:22:53 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        kernel list <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>, Peter Anvin <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: v4.10: kernel stack frame pointer .. has bad value (null)
Message-ID: <20170308212253.GA29562@amd>
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Hi!

> > - CONFIG_FUNCTION_GRAPH_TRACER sets it on x86-32 because of a gcc bug
> >   where the stack gets aligned before the mcount call.  This issue
> >   should be mostly obsolete as most modern compilers now have -mfentry.
> >   We could make it dependent on CC_USING_FENTRY.
>=20
> Yeah. At some point we might even upgrade the compiler requirements to
> no longer accept the mcount model.
>=20
> I think the fentry model is gcc-4.6.0 and up. Currently I guess we
> support gcc-3.2+, which is fairly ridiculous considering that 4.6.0 is
> from March, 2011. So it's over five years ago already.
>=20
> gcc-3.2.0 is from 2002, I think. At some point you just have to say
> "caring about a 15 year old compiler is ridiculous"
>=20
> The main reason we have fairly aggressively supported old compilers
> tends to be some odder architectures that don't have good support, so
> people use various random "this works for me" versions.
>=20
> We could easily make the gcc version checks much more strict on x86,
> I suspect.

Well, I have fast CPUs, but most of the time they just compile
stuff. Especially bisect is compile-heavy. I suspect going back to
gcc-3.2 would bring me bigger advantages than CPU upgrade...

								Pavel
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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