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

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Message-ID: <1348580900.11229.19.camel@zakaz.uk.xensource.com>
Date:	Tue, 25 Sep 2012 14:48:20 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
CC:	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, 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 ;-)

Ian.


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