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

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Date:	Tue, 21 Jul 2015 12:29:21 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Cc:	Jason Baron <jasonbaron0@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel broken on processors without performance counters

On Tue, Jul 21, 2015 at 12:00 PM,  <Valdis.Kletnieks@...edu> wrote:
> On Tue, 21 Jul 2015 11:54:30 -0700, Andy Lutomirski said:
>
>> Could this be done at link time, or perhaps when compressing the
>> kernel image, instead of at boot time?
>
> That's only safe to do if the kernel is built for one specific CPU - if it's
> a generic kernel that boots on multiple hardware designs, it will be wrong
> for some systems.
>
> In other words - safe to do if you're building it for *your* hardware. Totally
> unsafe for a distro.

That's not what I meant.  We do something in the C code that tells the
build step which way the initial state goes.  At link time, we make
the initial state actually work like that.  Then, at run time, we can
still switch it again if needed.

--Andy
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