From: Jeremy Fitzhardinge <jeremy@goop.org>
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>
Subject: Re: rdtsc hypercall, from userland?!? (was: rdtsc: correctness vs performance on Xen)
Date: Wed, 09 Sep 2009 11:59:11 -0700 [thread overview]
Message-ID: <4AA7FAFF.20505@goop.org> (raw)
In-Reply-To: <6dfe825c-f49e-402e-81d0-e8eaee3c9cda@default>
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
next prev parent reply other threads:[~2009-09-09 18:59 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-09-09 18:05 rdtsc hypercall, from userland?!? (was: rdtsc: correctness vs performance on Xen) Dan Magenheimer
2009-09-09 18:59 ` Jeremy Fitzhardinge [this message]
2009-09-10 9:59 ` Jan Beulich
2009-09-10 10:04 ` Jan Beulich
2009-09-10 10:14 ` Ian Pratt
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