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

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Date:	Wed, 26 Sep 2012 07:29:56 -0400
From:	Konrad Rzeszutek Wilk <konrad@...nel.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Woodhouse <dwmw2@...radead.org>,
	Vivek Goyal <vgoyal@...ibm.com>,
	Andres Salomon <dilinger@...ued.net>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH] x86-32: Start out eflags and cr4 clean

On Tue, Sep 25, 2012 at 02:48:20PM +0100, Ian Campbell wrote:
> On Tue, 2012-09-25 at 00:27 +0100, H. Peter Anvin wrote:
> > From: "H. Peter Anvin" <hpa@...ux.intel.com>
> > 
> > %cr4 is supposed to reflect a set of features into which the operating
> > system is opting in.  If the BIOS or bootloader leaks bits here, this
> > is not desirable.  Consider a bootloader passing in %cr4.pae set to a
> > legacy paging kernel, for example -- it will not have any immediate
> > effect, but the kernel would crash when turning paging on.
> > 
> > A similar argument applies to %eflags, and since we have to look for
> > %eflags.id being settable we can use a sequence which clears %eflags
> > as a side effect.
> > 
> > Note that we already do this for x86-64.
> > 
> > I would like opinions on this especially from the PV crowd
> 
> Xen PV guests don't pass through this code path so there is no danger
> there AFAICT, so from that PoV:
> 
> Acked-by: Ian Campbell <ian.campbell@...rix.com>
> 
> FWIW it looks correct to me from the native PoV too, but you probably
> already knew that ;-)

And sanity testing confirmed it.

Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> 
> Ian.
> 
> 
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