From: Yin FengweiThanks a lot for the comments.
Sent: 15 October 2019 09:04
In function acpi_idle_do_entry(), an ioport access is used for dummy
wait to guarantee hardware behavior. But it could trigger unnecessary
vmexit in virtualization environment.
If we run linux as guest and export all available native C state to
guest, we did see many PM timer access triggered VMexit when guest
enter deeper C state in our environment (We used ACRN hypervisor
instead of kvm or xen which has PM timer emulated and exports all
native C state to guest).
According to the original comments of this part of code, io port
access is only for dummy wait. We could use busy wait instead of io
port access to guarantee hardware behavior and avoid unnecessary
You need some hard synchronisation instruction(s) after the inb()
and before any kind of delay to ensure your delay code is executed
after the inb() completes.
I'm pretty sure that inb() is only synchronised with memory reads.
I did some testing. ktime_get_real_ts64() takes much less time than io
+ /* profiling the time used for dummy wait op */
That could be dominated by the cost of ktime_get_real_ts64().
It also need synchronising instructions.
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)