linux-kernel - Re: [PATCH v3] x86: let 'reservetop' functioning right

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Date:	Thu, 08 Apr 2010 20:41:24 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Liang Li <liang.li@...driver.com>
CC:	linux-kernel@...r.kernel.org, wangchen@...fujitsu.com,
	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	yinghai@...nel.org, akpm@...ux-foundation.org,
	jeremy.fitzhardinge@...rix.com, konrad.wilk@...cle.com,
	Alok Kataria <akataria@...are.com>
Subject: Re: [PATCH v3] x86: let 'reservetop' functioning right

On 04/08/2010 08:23 PM, Liang Li wrote:
> On Thu, Apr 08, 2010 at 08:12:18PM -0700, Jeremy Fitzhardinge wrote:
>   
>> On 04/08/2010 05:43 PM, Liang Li wrote:
>>     
>>> When specify 'reservetop=0xbadc0de' kernel parameter, the kernel will
>>> stop booting due to a early_ioremap bug that relate to commit 8827247ff.
>>>
>>> The root cause of boot failure problem is the value of 'slot_virt[i]'
>>> was initialized in setup_arch->early_ioremap_init. But later in
>>> setup_arch, the function 'parse_early_param' will modify 'FIXADDR_TOP'
>>> when 'reservetop=0xbadc0de' being specified.
>>>
>>> The simplest fix might be use __fix_to_virt(idx0) to get updated value
>>> of 'FIXADDR_TOP' in '__early_ioremap' instead of reference old value
>>> from slot_virt[slot] directly.
>>>   
>>>       
>> While I guess this patch works OK, I have to say that I'm worried by the
>> need for it at all; it seems to be papering over a more serious
>> problem.  reserve_top_address() is supposed to be called very early,
>> before anything has used or referenced FIXADDR_TOP.  If we're seeing
>> problems with FIXADDR_TOP changing after it has been used, then it means
>> that reserve_top_address() is being called too late.  Fixing that would
>> be the real fix.
>>     
> The ideal thing is FIXADDR_TOP should not be touched after
> early_ioremap_init. The late call to reserve_top_address is from
> parse_reservetop, aka when reservetop=0xabcd0000 being passed as kernel
> commandline parameter. In setup_arch, the call sequence is:
>
> setup_arch
>   -> early_ioremap_init
>   -> parse_early_param
>     -> parse_reservetop
> 	  ->reserve_top_address
>
> See, how could we solve the confliction better?
>   

Well, the first question is "do we need the reservetop= kernel
parameter"?  Zach added it, I think, so that VMI could be loaded
dynamically as a module.  Given that VMI is deprecated anyway, I wonder
if we can just drop support for modular VMI and remove the reservetop=
kernel parameter.  Or are there other uses for it?

    J
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