linux-kernel - Re: [RFC PATCH 04/21] x86/hweight: Add stack frame dependency for __arch

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Message-ID: <20150718050536.GA23478@nazgul.tnic>
Date:	Sat, 18 Jul 2015 07:05:36 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, x86@...nel.org,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 04/21] x86/hweight: Add stack frame dependency for
 __arch_hweight*()

On Fri, Jul 17, 2015 at 12:32:20PM -0500, Josh Poimboeuf wrote:
> Well, but this isn't some whitelist code to make stackvalidate happy.
> 
> It's actually a real runtime frame pointer bug, and the rsp dependency
> is real.  If it does the call without first creating the stack frame
> then it breaks frame pointer based stack traces.

I think we can live with the stack trace being a little wrong in those
__sw_* variants. And besides, we're talking about the very very small
percentage of machines (which keeps getting smaller) which don't
support POPCNT. And from those, only for the cases where the arg is not
__builtin_constant_p() because there we do the __const_hweight* thing.

I'd prefer to not clutter the code more in that case.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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