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    SubjectRe: [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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