On Fri, May 24, 2019 at 5:05 PM Fenghua Yu <[email protected]> wrote: > > umwait or tpause allows processor to enter a light-weight > power/performance optimized state (C0.1 state) or an improved > power/performance optimized state (C0.2 state) for a period > specified by the instruction or until the system time limit or until > a store to the monitored address range in umwait. > > IA32_UMWAIT_CONTROL MSR register allows kernel to enable/disable C0.2 > on the processor and set maximum time the processor can reside in > C0.1 or C0.2. > > By default C0.2 is enabled so the user wait instructions can enter the > C0.2 state to save more power with slower wakeup time. > > Default maximum umwait time is 100000 cycles. A later patch provides > a sysfs interface to adjust this value.
Reviewed-by: Andy Lutomirski <[email protected]> with the caveat that we should really clean up our CPU init code to have a function like cpu_prepare_for_user_code() that is called on all CPUs after every boot, resume, etc before running user code. This would subsume syscall_init(). --Andy
