Re: Question about running a program(Intel PCM) in ring 0 on Xen

[Dario Faggioli <dario.faggioli@citrix.com>]

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On lun, 2014-02-17 at 17:32 -0500, Meng Xu wrote:
> Hi,
> 
Hi,

> I'm a PhD student, working on real time system. 
> 
Cool. There really seems to be a lot of interest in Real-Time
virtualization these days. :-D

> [My goal]
> I want to measure the cache hit/miss rate of each guest domain in Xen.
> I may also want to measure some other events, say memory access rate,
> for each program in each guest domain in Xen.
> 
Ok. Can I, out of curiosity, as you to detail a bit more what your
*final* goal is (I mean, you're interested in these measurements for a
reason, not just for the sake of having them, right?).

> [The problem I'm encountering]
> I tried intel's Performance Counter Monitor (PCM) in Linux on bare
> machine to get the machine's cache access rate for each level of
> cache, it works very well. 
> 
> 
> However, when I want to use the PCM in Xen and run it in dom0, it
> cannot work. I think the PCM needs to run in ring 0 to read/write the
> MSR. Because dom0 is running in ring 1, so PCM running in dom0 cannot
> work. 
> 
Indeed.

> So my question is:
> How can I run a program (say PCM) in ring 0 on Xen? 
> 
Running "a program" in there is going to be terribly difficult. What I
think you're better off is trying to access, from dom0 and/or
(para)virtualize the counters.

In fact, there is work going on already on this, although I don't have
all the details about what's the current status.

> What's in my mind is:
> Writing a hypercall to call the PCM in Xen's kernel space, then the
> PCM will run in ring 0? 
> But the problem I'm concerned is that some of the PCM's instruction,
> say printf(), may not be able to run in kernel space? 
> 
Well, Xen can print, e.g., on a serial console, but again, that's not
what you want. I'm adding the link to a few conversation about virtual
PMU. These are just the very first google's result, so there may well be
more:

http://xen.1045712.n5.nabble.com/Virtualization-of-the-CPU-Performance-Monitoring-Unit-td5623065.html
https://lwn.net/Articles/566159/

Boris (which I'm Cc-ing), gave a presentation about this at latest Xen
Developers Summit:
http://www.slideshare.net/xen_com_mgr/xen-pmu-xensummit2013

Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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