From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <ian.campbell@citrix.com>
Subject: Re: [PATCH v4] x86/viridian: Add Partition Reference
 Time enlightenment
Date: Tue, 14 Oct 2014 11:50:36 +0100
Message-ID: <1413283836.10417.36.camel@citrix.com>
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To: Paul Durrant <paul.durrant@citrix.com>
Cc: Keir Fraser <keir@xen.org>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Christoph Egger <chegger@amazon.de>, Ian Jackson <ian.jackson@eu.citrix.com>, Jan Beulich <jbeulich@suse.com>, xen-devel@lists.xenproject.org
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On Tue, 2014-10-14 at 11:45 +0100, Paul Durrant wrote:
> The presence of the partition reference time enlightenment persuades newer
> versions of Windows to prefer the TSC as their primary time source. Hence,
> if rdtsc is not being emulated and is invariant then many vmexits (for
> alternative time sources such as the HPET or reference counter MSR) can
> be avoided.
> 
> The implementation is not yet complete as no attempt is made to prevent
> emulation of rdtsc if the enlightenment is active and guest and host
> TSC frequencies differ. To do that requires invasive changes in the core
> x86 time code and hence a lot more testing.
> 
> This patch avoids the issue by disabling the enlightenment if rdtsc is
> being emulated, causing Windows to choose another time source. This is
> safe, but may cause a big variation in performance of guests migrated
> between hosts of differing TSC frequency. Thus the enlightenment is not
> enabled in the default set, but may be enabled to improve guest performance
> where such migrations are not a concern.
> 
> See section 15.4 of the Microsoft Hypervisor Top Level Functional
> Specification v4.0a for details.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> Cc: Keir Fraser <keir@xen.org>
> Cc: Jan Beulich <jbeulich@suse.com>

For tools/*

Acked-by: Ian Campbell <ian.campbell@citrix.com>