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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 07 January 2015 10:08
> To: Paul Durrant
> Cc: Christoph Egger; Ian Campbell; Ian Jackson; Stefano Stabellini; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [PATCH v4] x86/viridian: Add Partition Reference Time
> enlightenment
> 
> >>> On 14.10.14 at 12:45, <paul.durrant@xxxxxxxxxx> 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>
> 
> This doesn't apply anymore and hence needs to be re-spun against
> current staging.

Ok. I'll rebase and repost.

  Paul

> 
> Jan


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