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

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Message-ID: <5220D350.7030008@zytor.com>
Date:	Fri, 30 Aug 2013 10:16:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 08/30/2013 10:03 AM, Steven Rostedt wrote:
> On Fri, 30 Aug 2013 09:35:18 -0700
> "H. Peter Anvin" <hpa@...or.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?


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