linux-kernel - Re: [Techteam] [RFC PATCH] x86-32: Start out eflags and cr4 clean

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Date:	Fri, 18 Jan 2013 18:35:13 -0800
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Mitch Bradley <wmb@...top.org>
CC:	Andres Salomon <dilinger@...ued.net>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Vivek Goyal <vgoyal@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, techteam@...ts.laptop.org,
	Yinghai Lu <yhlu.kernel@...il.com>
Subject: Re: [Techteam] [RFC PATCH] x86-32: Start out eflags and cr4 clean

On 01/18/2013 05:05 PM, Mitch Bradley wrote:
> 
> 
> On 1/18/2013 2:42 PM, H. Peter Anvin wrote:
>> On 01/18/2013 04:40 PM, Andres Salomon wrote:
>>> Bad news on this patch; I've been told that it breaks booting on an
>>> XO-1.5.  Does anyone from OLPC know why yet?
>>
>> What are the settings of CR0 and CR4 on kernel entry on XO-1.5?
> 
> 
> CR0 is 0x80000011
> CR4 is 0x10
> 

OK, that makes sense... the kernel doesn't enable the PSE bit yet and I
bet that's what you're using for the non-stolen page tables.

Can we simply disable paging before mucking with CR4?  The other option
that I can see is to always enable PSE and PGE, since they are simply
features opt-ins that don't do any harm if unused.  At the same time,
though, entering the kernel through the default_entry path with paging
enabled is definitely not anything the kernel expects.

Does this patch work for you?  Since we have ditched 386 support, we can
mimic x86-64 (yay, one more difference gone!) and just use a predefined
value for %cr0 (the FPU flags need to change if we are on an FPU-less
chip, but that happens during FPU probing.)

Does this patch work for you?

	-hpa

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