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Message-ID: <1348580900.11229.19.camel@zakaz.uk.xensource.com>
Subject: Re: [RFC PATCH] x86-32: Start out eflags and cr4 clean
From: Ian Campbell <Ian.Campbell@citrix.com>
To: "H. Peter Anvin" <hpa@linux.intel.com>
CC: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Yinghai Lu <yhlu.kernel@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>
Date: Tue, 25 Sep 2012 14:48:20 +0100
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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 ;-)

Ian.