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.