linux-kernel - Re: Kernel broken on processors without performance counters

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Date:	Thu, 23 Jul 2015 10:54:43 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Baron <jasonbaron0@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Vince Weaver <vince@...ter.net>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: Kernel broken on processors without performance counters

On Tue, 21 Jul 2015 12:57:08 -0400
Jason Baron <jasonbaron0@...il.com> wrote:


> On 07/21/2015 12:12 PM, Peter Zijlstra wrote:
> > On Tue, Jul 21, 2015 at 08:51:51AM -0700, Andy Lutomirski wrote:
> >> To clarify my (mis-)understanding:
> >>
> >> There are two degrees of freedom in a static_key.  They can start out
> >> true or false, and they can be unlikely or likely.  Are those two
> >> degrees of freedom in fact tied together?
> > Yes, if you start out false, you must be unlikely. If you start out
> > true, you must be likely.
> >
> > We could maybe try and untangle that if there really is a good use case,
> > but this is the current state.
> 
> We could potentially allow all combinations but it requires a more
> complex implementation. Perhaps, by rewriting no-ops to jmps
> during build-time? Steve Rostedt posted an implementation a while
> back to do that, but in part it wasn't merged due to its
> complexity and the fact that it increased the kernel text, which
> I don't think we ever got to the bottom of. Steve was actually

I think that happened because we didn't save enough with the nops to
compensate for the code that it took to implement that change. Perhaps
in the future that will be different as the implementation is a fixed
size, while the usage of jump labels increase.

-- Steve

> trying to reduce the size of the no-ops by first letting the compiler
> pick the 'jmp' instruction size (short jumps of only 2-bytes on
> x86, instead of the hard-coded 5 bytes we currently have).
> However, we could use the same idea here to allow the mixed
> branch label and initial variable state.
> 
> That said, it seems easy enough to reverse the direction of
> the branch in an __init or so when we boot, if required.
> 

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