From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: rdtscP and xen (and maybe the app-tsc answer I've
	been looking for)
Date: Tue, 22 Sep 2009 12:52:41 -0700
Message-ID: <4AB92B09.8020308@goop.org>
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To: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: kurt.hackel@oracle.com, "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 09/22/09 12:36, Dan Magenheimer wrote:
> Are you (Jeremy) still assuming that rdtsc/rdtscp are NOT
> emulated?  Or are you trying to define a vsyscall+pvclock
> mechanism for the same constrained environments
> so that HFTSAs have a choice of using clock_gettime
> instead of pvrdtsc, either of which will be fast?
>   

Yes, I'm assuming they're not emulated.  If you're emulating them
there's no reason to add any extra complexity to usermode by adding any
other ABI: rdtsc can be rdtsc and rdtscp can be rdtscp with no
Xen/ABI-imposed constraints on TSC_AUX.

Once you're talking about layering another ABI onto the tsc, then
there's no need to consider emulation because you can do all the
necessary correction to get a canonical timestamp without it.

    J