Messages in this thread | | | Date | Thu, 6 Jan 2022 11:29:19 +0800 | From | Like Xu <> | Subject | Re: [PATCH v2] KVM: X86: Emulate APERF/MPERF to report actual vCPU frequency |
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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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