linux-kernel - Re: [PATCH v2 1/3] x86, boot: add mmio serial during compressed boot

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Message-ID: <CAE9FiQUAGPgqU3vOOjKJva=nN71CKoOEdpyjg_23-OfgFSkG=A@mail.gmail.com>
Date:	Tue, 16 Jul 2013 14:03:54 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Landley <rob@...dley.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v2 1/3] x86, boot: add mmio serial during compressed boot

On Tue, Jul 16, 2013 at 12:40 PM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Jul 16, 2013 at 12:36 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 07/16/2013 12:31 PM, Kees Cook wrote:
>>>
>>> Could the first step be documenting the limitation? I've found this
>>> patch extremely useful for my case already, and I imagine there might
>>> be other people that need the early mmio stuff to. Generally the
>>> compressed boot serial console stuff is going to be used in the more
>>> common non-kexec situations at least for a while.
>>>
>>> Does this patch create any _problems_? Right now, neither low nor >4G
>>> kernel can use an mmio serial port. :) This this, we'd at least gain
>>> it for the low case.
>>>
>>
>> Even documenting the limitation is likely to end up with a bunch of
>> emails asking why their kernel crashed.
>>
>> I think setting up a dynamic #PF handler is the right thing for the
>> decompressor, we already did for the kernel proper.
>
> I'm not sure how to accomplish this yet. I'm still trying to
> understand how the page tables are arranged. :)

Other way could be:

Detect if it get into misc.c directly from
arch/x86/boot/compressed/head_64.S::startup_64. that is from 64bit
bootloader.

others go through startup_32 should be 32bit bootloader.

If it with 64bit boot loader path, You can skip parsing mmio in
misc.c::console_init() calling.

Yinghai
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