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Date: Fri, 30 Aug 2013 10:16:00 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Steven Rostedt <rostedt@goodmis.org>
CC: Jan Kiszka <jan.kiszka@siemens.com>, Fenghua Yu <fenghua.yu@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: x86-32: Early microcode loading stumbles over disabled DYNAMIC_FTRACE
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On 08/30/2013 10:03 AM, Steven Rostedt wrote:
> On Fri, 30 Aug 2013 09:35:18 -0700
> "H. Peter Anvin" <hpa@zytor.com> wrote:
> 
>> On 08/30/2013 08:51 AM, Steven Rostedt wrote:
>>>
>>> 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.
>>>
>>
>> And of course global variables are a no-no without special handling in
>> pre-paging mode.  Sigh.
>>
>> You *could* bail out of the calling address is < PAGE_OFFSET.
>>
> 
> We could do that too, as static tracing already gives us high overhead,
> I'm not sure how much more overhead another check like this even
> matters.

It's not too expensive: a compare and a jump.

> Who uses static tracing? I only use it to test that it still works ;-)

Can we get rid of it?