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Re: [Xen-devel] [PATCH] vmx: last branch recording MSR emulation

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] vmx: last branch recording MSR emulation
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Thu, 09 Aug 2007 14:20:23 +0100
Cc: Xin B Li <xin.b.li@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On 9/8/07 14:05, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> The default of reading native registers is of course very questionable, but
> it's been that way for so long that I didn't dare to kill this, as I'm
> suspecting
> that booting some, if not all, OSes without this would not work. And the then
> likely resulting incrementally adding of emulation for individual MSRs on an
> empirical basis is something that I consider, as pointed out on other similar
> occasions like the MMIO instruction decoder, very wrong for code that is no
> longer in a proof-of-concept state.

Doing it the current way you get bitten when host MSRs contain unexpected
values. This is why the MCE/MCA MSR handling got added -- some guests were
getting confused, and thius crashing, on one particular host type that we
hadn't tested on before. We're just storing up trouble for the future.

 -- Keir

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