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Re: [Xen-devel] [PATCH 2 of 3] apic: remove 'enabled_via_apicbase' varia

To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2 of 3] apic: remove 'enabled_via_apicbase' variable
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Wed, 18 May 2011 16:45:37 -0400
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References: <patchbomb.1305742093@andrewcoop> <e80b5280fe2fb8653204.1305742095@andrewcoop> <20110518185339.GD14013@xxxxxxxxxxxx> <4DD42D9E.7030700@xxxxxxxxxx>
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> >So if you don't have x2apic, then it is wrong to disable the LAPIC mode?
> >What about older hardware?
> I guess I wasn't very clear in my description.  In older hardware
> without x2apic, it is correct to simply twiddle the ENABLE bit in
> the APICBASE MSR.  However, with x2apic mode enabled, setting the
> ENABLE bit from 1 to 0 while leaving the EXTD bit set will result in
> a protection fault which will propagate to a general protection
> fault the same codepath will be called in the fault handler.  As a
> result, the current code in disable_local_APIC will result in a GPF
> if the BIOS boots with LAPICs disabled (fine as per the spec for
> compatibility) and xen decided to take advantage of x2apic mode.

Ok. You might also point to the appropiate section of the x2APIC
spec about this. I think it is

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