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    SubjectRe: [PATCH v2] KVM: X86: Emulate APERF/MPERF to report actual vCPU frequency
    On 6/1/2022 6:51 am, Jim Mattson wrote:
    > On Thu, Dec 30, 2021 at 11:48 PM Like Xu <like.xu.linux@gmail.com> wrote:
    >>
    >> On 31/12/2021 9:29 am, Jim Mattson wrote:
    >
    >>> At sched-in:
    >>> 1. Save host APERF/MPERF values from the MSRs.
    >>> 2. Load the "current" guest APERF/MPERF values into the MSRs (if the
    >>> vCPU configuration allows for unintercepted reads).
    >>>
    >>> At sched-out:
    >>> 1. Calculate the guest APERF/MPERF deltas for use in step 3.
    >>> 2. Save the "current" guest APERF/MPERF values.
    >>> 3. "Restore" the host APERF/MPERF values, but add in the deltas from step 1.
    >>>
    >>> Without any writes to IA32_MPERF, I would expect these MSRs to be
    >>> synchronized across all logical processors, and the proposal above
    >>> would break that synchronization.
    >
    > I am learning more about IA32_APERF and IA32_MPERF this year. :-)

    Uh, thanks for your attention.

    >
    > My worry above is unfounded. These MSRs only increment in C0, so they
    > are not likely to be synchronized.
    >
    > This also raises another issue with your original fast-path
    > implementation: the host MSRs will continue to count while the guest
    > is halted. However, the guest MSRs should not count while the guest is
    > halted.
    >

    The emulation based on guest TSC semantics w/ low precision may work it out.
    TBH, I still haven't given up on the idea of a pass-through approach.

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