linux-kernel - Re: [PATCH v7 06/27] x86, 64bit: early #PF handler set page table

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Date:	Thu, 20 Dec 2012 08:56:11 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	Jan Kiszka <jan.kiszka@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 06/27] x86, 64bit: early #PF handler set page table

On Tue, Dec 18, 2012 at 1:07 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 12/18/2012 12:55 PM, Yinghai Lu wrote:
>> On Tue, Dec 18, 2012 at 12:49 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 12/18/2012 12:43 PM, Yinghai Lu wrote:
>>
>>>
>>> That is putting the cart before the horse.  What is the specific requirement
>>> with kgdb here (I didn't see any email on that, please don't have private
>>> back conversations)?  Either way, however, kgdb is a tool to debug the
>>> kernel... having it a barrier for proper functionality of the kernel is not
>>> acceptable.
>>
>> did not hear back from Jason or Jan.
>>
>> Looks like last mail in LKML from Jason is about Oct 20
>>
>> looks like kgdb is want DB, BP, and PF are set at first.
>>
>> and just after that early_param for kgdbwait will get into to hold the kernel.
>>
>> then command from kgdb could dump ram etc.
>>
>
> So in that case, kgdb is broken and will need to be fixed up.  That
> happens all the time with debugging tools.
>

If there is a way that we can make all parties happy, we really should
not break KGDB.

Please reconsider to stop #PF handler in x86_64_start_kernel. in that case
1. microcode update still can use #PF handler to find microcode in
ramdisk and use it.
2. kernel that is loaded above 4G, could set mapping in C instead of
set that in head_64.S
   and use ioremap to access zero_page
3. KGDB still can call early_trap_init early before init_mem_mapping.

I put the change in for-x86-boot-v8 branch.
core patch is:
http://git.kernel.org/?p=linux/kernel/git/yinghai/linux-yinghai.git;a=commitdiff;h=6fa4f1e68f0b67d0dc13d30c5ce6c3932697d08f

Thanks

Yinghai
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