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

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Message-ID: <ee05bb34-7671-3069-3eb0-f794a2dc68a4@oracle.com>
Date:   Wed, 2 May 2018 11:06:22 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jan Beulich <JBeulich@...e.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 05/02/2018 04:26 AM, Jan Beulich wrote:
>>>> 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).

How would keeping __BOOT_XX selectors help with NMI? We don't have
anything set up for NMI handling anyway yet, this is all done in x86
startup code later.

-boris