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[Xen-devel] Re: [PATCH/RFC] report hardware tsc frequency even for emula

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH/RFC] report hardware tsc frequency even for emulated tsc
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 30 Nov 2009 11:21:37 -0800
Cc: "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On 11/30/09 07:55, Dan Magenheimer wrote:
> This patch changes the reported hz rate to the
> hz rate of the initial machine on which the guest
> is booted and retains that reported hz rate across
> save/restore/migration.

The pvclock algorithm relies on knowing what rate the tsc advances at. 
If you return  the "real" tsc speed in the clock info, won't that break?

> Jeremy has pointed out that reporting 1000.0 MHz is
> useful because it shows that TSC is being emulated.
> However, with the new tsc_mode default where
> a guest may start with native TSC and switch to
> emulated TSC after migration, users are likely to
> get even more confused.

I don't see how the original CPU speed is any more relevant than the
emulated speed, which is at least constant.  The CPU speed is fairly
pointless info given that it has no bearing on how fast the CPU is
running from moment to moment, even ignoring migration.  (On native "cpu
MHz" updates on the fly as the CPU speed changes, but I don't think your
proposal will allow that.)

>   And "xm debug-key s"
> reveals not only whether TSC is being emulated but
> also the frequency so is more descriptive anyway.

It is not available to non-privileged users.


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