linux-kernel - Re: [Xen-devel] [PATCH 4/4] xen/PVH: Remove reserved entry in PVH GDT

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Message-Id: <5AE9764D02000078001C00BE@prv1-mh.provo.novell.com>
Date:   Wed, 02 May 2018 02:26:53 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Cc:     "Roger Pau Monne" <roger.pau@...rix.com>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 4/4] xen/PVH: Remove reserved entry in
 PVH GDT

>>> On 01.05.18 at 14:34, <boris.ostrovsky@...cle.com> wrote:
> On 05/01/2018 04:00 AM, Roger Pau Monné wrote:
>> On Mon, Apr 30, 2018 at 12:23:39PM -0400, Boris Ostrovsky wrote:
>>> And without it we can't use _BOOT_XX macros any longer so define new ones.
>> 
>> Not being that familiar with Linux internals I'm not sure I see the
>> benefit of this. Isn't there a risk that some other code is going to
>> use the __BOOT_XX defines?
> 
> The startup code we are jumping to loads their own GDT and I don't see 
> any explicit references to segments.

No explicit references to segments isn't enough: You also need to make
sure no exceptions at all can occur while loaded selectors and GDT are
out of sync - in particular NMI might be of concern here (this isn't PV
after all, where not having a callback registered effectively masks NMI).

Jan