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Re: [Xen-devel] APIC MSRs query

To: Jan Beulich <JBeulich@xxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Subject: Re: [Xen-devel] APIC MSRs query
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Tue, 17 May 2011 15:53:46 +0100
Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] APIC MSRs query
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On 17/05/2011 15:08, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

>>> I think this is a benign mistake. Benign because I don't think there is
>>> a meaningful (to Xen at least) number of systems that would not
>>> have their LAPIC at the default address (which fits in 32 bits).
>> That "msr_content = (uint32_t)msr_content;" seems to be pretty
>> deliberate, what else would it be trying to do?
>> FWIW enable_x2apic in Linux seems to have a similar construct which
>> throws away the top half of the MSR:
> Surely the Xen code got cloned from the Linux one.

If you actually look at the change in 21669:b622e411, you can see that the
above line is deliberate. The old code did wrmsr(msr, lo, 0), and clearing
upper 32 bits before calling wrmsrl() is the equivalent of that.

So this should not be 'cleaned up' without understanding that old code
(which almost certainly itself comes from Linux).

And I should also warn that I will simply ignore gratuitous cleanups in the
APIC code. The code is mostly Linux sourced, I'd rather we synced again than
diverge further.

 -- Keir

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