linux-kernel - Re: x86-32: Early microcode loading stumbles over disabled DYNAMIC

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Message-ID: <20130830115142.1ee4c825@gandalf.local.home>
Date:	Fri, 30 Aug 2013 11:51:42 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	x86 <x86@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: x86-32: Early microcode loading stumbles over disabled
 DYNAMIC_FTRACE

On Fri, 30 Aug 2013 08:36:58 -0700
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 08/23/2013 09:40 AM, Jan Kiszka wrote:
> > Hi all,
> > 
> > 32-bit kernels currently crash/reboot in early microcode loading when
> > non-dynamic function tracing is enabled. Several functions in that path
> > get instrumented with mcount, but its non-dynamic implementation does
> > not work before paging is enabled (it accesses global variables at wrong
> > addresses).
> > 
> > Below some hunks to get it working again - at least in the absence of
> > any microcode in the initrd. Marking all involved functions as __init is
> > another option (as __init implies notrace). But I bet there is more
> > hidden. I see e.g. a pr_warn() in find_cpio_init that should trigger the
> > issue as well if we hit the error it reports (btw. printing at this
> > point of the boot should not work anyway, should it?).
> > 
> > Better ideas?
> > 
> > Jan
> > 
> 
> A better idea would be for the mcount/__fentry__ function to simply
> return until the function tracing stuff is actually ready.
> 

It does when dynamic tracing is enabled. But this issue is with static
tracing, there's no code modification, thus all functions call
mcount/__fentry__ and we need to look at a variable to determine if we
should trace or not.

-- Steve
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