Re: [Xen-users] perf hardware event on ARM64

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Sun, 2015-10-25 at 18:51 -0400, Jintack Lim wrote:
> 
> 
> On Fri, Oct 23, 2015 at 9:44 AM, Ian Campbell <ian.campbell@xxxxxxxxxx>
> wrote:
> > On Fri, 2015-10-23 at 09:22 -0400, Jintack Lim wrote:
> > > Hi,
> > >
> > > I'm working on ARM64 (AppliedMicroâ X-Gene).
> > > I wanted to run perf to collect some hardware events, but I got some
> > > trouble.
> > > Basically I have two questions.
> > >
> > > 1.
> > > I found that pmu is not registered in the device tree,
> > > while it is registered on native linux (installed on the same type of
> > > machine) .
> > > Can I just modify device tree at booting time to make it look like
> > native
> > > linux?
> > > Will it work?
> > 
> > > Additional information is at the bottom.
> > >
> > > 2.
> > > I found that running perf on Xen 4.5.0 resulted in segmentation fault
> > (or
> > > system crash),
> > > however it runs well on the latest Xen on git server.
> > > Can somebody point the patch that makes this change?
> > 
> > 4.5.0 was vulnerable to XSA-93:
> > 
> > http://xenbits.xen.org/xsa/advisory-93.html
> > 
> > The core issue with PMU is that Xen currently does not know how to context
> > switch the relevant state when changing guests. Fixing that would be more
> > work than just fiddling with device tree.
> > 
> > You might be able to bodge something useful (but unsupported) together by
> > just allowing only dom0 to access these registers and to see the values
> > collected from the whole system (i.e. don't worry about context switching).
> > How useful such numbers would be depends somewhat on what you are hoping to
> > find out from this exercise.
> My machine has 8 cores,
> and I'm running dom0 on core 0~3, and domU on 4~7.
> I wonder if running perf on dom0 can profile hardware events on cores, on
> which dom0 is not running.

Please remember: performance counter use from guests is not something
currently supported by Xen on ARM, so perf on dom0 cannot, in a formally
supported sense, profile anything at all.

If you are talking about using the bodge mentioned above then of course it
can only profile things running on processors to which it has (pinned, 1:1)
access and there may be other more serious caveats I've not thought of
because this is not something which is supported.

Ian.


> If not, then I should run perf on domU I guess.
>  
> >  
> > Ian.
> > 
> > 
> > 

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