Does xen support Intel Processor Trace?

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Does xen support Intel Processor Trace?

Zihan Yang
I'd like to use Intel PT to analyze xen crashes, but I don't find the option in the code. Is it supported by Xen? If not, is there any plan to do so?

Thanks

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Re: Does xen support Intel Processor Trace?

Roger Pau Monné-3
On Wed, Sep 13, 2017 at 10:47:03PM -0400, Zihan Yang wrote:
> I'd like to use Intel PT to analyze xen crashes, but I don't find the

I'm not sure Intel PT is useful to analyze Xen crashes. If Xen crashes
the whole system goes down, so how are you going to fetch the trace
data then?

It could be interesting for performance analysis IMHO.

> option in the code. Is it supported by Xen? If not, is there any plan to do
> so?

You would have to ask the Intel guys.

Thanks, Roger.

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Re: Does xen support Intel Processor Trace?

Zihan Yang
> > I'd like to use Intel PT to analyze xen crashes, but I don't find the
>
> I'm not sure Intel PT is useful to analyze Xen crashes. If Xen crashes
> the whole system goes down, so how are you going to fetch the trace
> data then?

It seems that Xen has already ported the kdump mechanism(in
http://www-archive.xenproject.org/files/summit_3/kexec_kdump.pdf).
So I guess it might be possible to coredump xen when it crashes, altough I have
not tried it myself. According to your experience, is it a practical solution?

> It could be interesting for performance analysis IMHO.

I agree, but pure instruction flow seems not sufficient to detect accurate
reason of performance downgrade(e.g. The growth of response delay). I think some
extra manual checks are needed.

Thanks
Zihan

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Re: Does xen support Intel Processor Trace?

Roger Pau Monné-3
On Wed, Sep 20, 2017 at 12:08:11AM -0400, Zihan Yang wrote:
> > >* I'd like to use Intel PT to analyze xen crashes, but I don't find the*
> >
> > I'm not sure Intel PT is useful to analyze Xen crashes. If Xen crashes
> > the whole system goes down, so how are you going to fetch the trace
> > data then?
> It seems that Xen has already ported the kdump mechanism(in
> http://www-archive.xenproject.org/files/summit_3/kexec_kdump.pdf).
> So I guess it might be possible to coredump xen when it crashes, altough I have
> not tried it myself. According to your experience, is it a practical solution?

In my experience the best way to debug Xen crashes is to use the
serial console and add debug messages when necessary.

> > It could be interesting for performance analysis IMHO.
>
> I agree, but pure instruction flow seems not sufficient to detect accurate
> reason of performance downgrade(e.g. The growth of response delay). I
> think some
> extra manual checks are needed.

I'm not sure what you mean with "The growth of response delay", but in
any case I guess Intel PT could be useful for performance analysis if
someone spends the time to port it to Xen.

Roger.

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