From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: rdtsc hypercall,
 from userland?!? (was: rdtsc: correctness vs performance on Xen)
Date: Wed, 09 Sep 2009 11:59:11 -0700
Message-ID: <4AA7FAFF.20505@goop.org>
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To: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: "Xen-Devel (E-mail)" <xen-devel@lists.xensource.com>, Keir Fraser <keir.fraser@eu.citrix.com>, Jan Beulich <JBeulich@novell.com>
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On 09/09/09 11:05, Dan Magenheimer wrote:
>  BUT
> (IMPORTANT NEW POINT!!!) the pvclock algorithm requires
> an rdtsc instruction, and there is no way to
> emulate some guest rdtsc instructions (e.g. only
> those in apps) and not others (e.g. only those in
> the kernel).  Thus, for guests that have rdtsc emulation
> enabled, vsyscall+pvclock will be SLOWER than emulation,
> thus meaning it is still not a palatable alternative.
>   

You could enable/disable emulation rdtsc each context switch according
to the app's desires/requrements.  It would require an extra hypercall
per context switch, but it could be batched with the others, resulting
in little marginal cost.  It would, however, leave the kernel's use of
rdtsc in a confused state.

    J