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Re: [Xen-devel] [PATCH v1 0/5] xen/PMU: PMU support for Xen PV guests



On 09/11/2013 05:33 AM, David Vrabel wrote:
On 10/09/13 16:31, Boris Ostrovsky wrote:
This is the Linux side of Xen PMU support for PV guests, including dom0. Only
kernel changes are here, toolstack patch will be provided separately.

Here is description from the hypervisor patch submission that applies to this
series as well:

This version has following limitations:
* For accurate profiling of dom0/Xen dom0 VCPUs should be pinned.
* Hypervisor code is only profiled on processors that have running dom0 VCPUs
on them.
* No backtrace support.
These are some pretty significant limitations.   Is there a plan for how
to remove them?

I don't have a specific plan (other than do it after this stage is finished) but I do
have a rough idea of what would be needed to address these.

Hypervisor changes for all three should be pretty easy.

Linux-wise, for the first one (pinned VCPU) we will probably need to make a
change in perf_sample_data. There is a reserved filed there so perhaps we
can use it to store PCPU (and maybe domainID). Plus a way to actually write
the data, probably a hook or something.

Backtrace support should also not be too bad: we can pass Xen IP stack in
the shared data area to dom0 and then again have a hook or something to
pass it to perf. (Note that backtracing is not supported for KVM neither so both
may benefit here)

The second one is the most difficult: We need to be able somehow to access
MSRs (and APIC, I think) on non-dom0 CPUs. Creating interface for such access is not a big deal but integrating it into perf infrastructure will be a challenge. There
are other alternatives but they have problems as well.


-boris

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