On Thu, Dec 30, 2021 at 11:48 PM Like Xu <like.xu.linux@xxxxxxxxx> 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. :-) 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.
