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Re: [Xen-devel] [PATCH v4] x86/viridian: Add Partition Reference Time enlightenment

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@xxxxxxxxxx>
> Cc: Keir Fraser <keir@xxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>

For tools/*

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

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