linux-kernel - Re: [PATCH] x86/microcode_intel_early.c: Get 32-bit physical address by __pa

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Message-ID: <CAE9FiQVHqt+Tx4vLuHQUQoZ-PcaGO4-oq7SjA5oq_M6mJh9DDQ@mail.gmail.com>
Date:	Tue, 19 Mar 2013 16:48:42 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	"Yu, Fenghua" <fenghua.yu@...el.com>
Cc:	H Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"penguin-kernel@...ove.sakura.ne.jp" 
	<penguin-kernel@...ove.sakura.ne.jp>,
	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH] x86/microcode_intel_early.c: Get 32-bit physical address
 by __pa_nodebug()

On Tue, Mar 19, 2013 at 4:36 PM, Yu, Fenghua <fenghua.yu@...el.com> wrote:
>> From: yhlu.kernel@...il.com [mailto:yhlu.kernel@...il.com] On Behalf Of
>> Yinghai Lu
>> On Tue, Mar 19, 2013 at 8:04 AM, Fenghua Yu <fenghua.yu@...el.com>
>> wrote:
>> > From: Fenghua Yu <fenghua.yu@...el.com>
>> >
>> > In 32-bit, __pa_symbol() in CONFIG_DEBUG_VIRTUAL accesses kernel data
>> (e.g.
>> > max_low_pfn) that haven't been setup yet in such early boot phase. To
>> fix the
>> > issue, __pa_nodebug() replaces __pa_symbol() to get a global symbol's
>> physical
>> > address.
>>
>> That explanation is not precise.
>>
>> __phys_addr in 32bit when DEBUG_VIRTUAL is set, it does have check
>>         /* max_low_pfn is set early, but not _that_ early */
>>         if (max_low_pfn) {
>>          ....
>>         }
>> the reason should be:
>> in head_32.S, 32bit flat mode, you need to switch to phys addr to
>> access
>> global variable max_low_pfn.
>
> Max_low_pfn is not set up yet at this point, but __pa_symbol() reads it
> if CONFIG_DEBUG_VIRTUAL. There is no need to access max_low_pfn or any
> other data that is not setup yet if CONFIG_DEBUG_VIRTUAL.
>
> So I think the explanation is ok.


Changlog said:

 In 32-bit, __pa_symbol() in CONFIG_DEBUG_VIRTUAL accesses kernel data
(e.g. max_low_pfn) that haven't been setup yet in such early boot phase.

---

So do you mean we have to change all __pa_symbol before
setup.c::setup_arch/find_low_pfn_range ?
because only at that time max_low_pfn get setup.
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