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

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Message-ID: <5148F2E9.5000401@zytor.com>
Date:	Tue, 19 Mar 2013 16:21:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 03/19/2013 04:15 PM, Yinghai Lu wrote:
> 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.
> 

Uh, no.

It already does that.  The problem is that __pa_symbol() breaks with
CONFIG_DEBUG_VIRTUAL at this point; we need to use __pa_nodebug().

	-hpa

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