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Date: Wed, 26 Sep 2012 07:29:56 -0400
From: Konrad Rzeszutek Wilk <konrad@kernel.org>
To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
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        "Eric W. Biederman" <ebiederm@xmission.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        Rusty Russell <rusty@rustcorp.com.au>,
        David Woodhouse <dwmw2@infradead.org>, Vivek Goyal <vgoyal@in.ibm.com>,
        Andres Salomon <dilinger@queued.net>,
        Yinghai Lu <yhlu.kernel@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [RFC PATCH] x86-32: Start out eflags and cr4 clean
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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@linux.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@citrix.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@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> 
> Ian.
> 
> 
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