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

[Jan Beulich]

>>> On 01.05.18 at 14:34, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 05/01/2018 04:00 AM, Roger Pau Monnà 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