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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-10-23 at 10:21 -0400, Jintack Lim wrote:
> Thanks Ian.
> 
> 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.
> So, if we don't worry about the context switching, then modifying device
> tree will just work?

You'd probably also need to adjust whichever register it is which causes
these accesses to be trapped to Xen.

> In my environment, I pinned vcpus of Dom0 and DomU on different set of 
> physical cores, 
> so there will be no context switch (except switching to idle domain)

OK. (although performance data from such a non-default configuration is
necessarily going to come with certain caveats)

BTW, any reason you aren't running 4.6 for this?

Ian.

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