Re: [PATCH v2] ACPI / processor_idle: use ndelay instead of io port access for wait

From: Yin Fengwei
Date: Tue Oct 22 2019 - 02:29:16 EST

On 2019/10/18 äå6:12, Rafael J. Wysocki wrote:
On Wednesday, October 16, 2019 7:56:17 AM CEST Yin, Fengwei wrote:
Hi David,

On 10/15/2019 7:48 PM, David Laight wrote:
From: Yin Fengwei
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.
Thanks a lot for the comments.

I didn't find the common serializing instructions API in kernel (only
memory barrier which is used to make sure of memory access). For Intel
x86, cpuid could be used as serializing instruction. But it's not
suitable for common code here. Do you have any suggestion?

In the virt guest case you don't need to worry at all AFAICS, because the inb()
itself will trap to the HV.

+ /* profiling the time used for dummy wait op */
+ ktime_get_real_ts64(&ts0);
+ inl(acpi_gbl_FADT.xpm_timer_block.address);
+ ktime_get_real_ts64(&ts1);

You may as well use ktime_get() for this, as it's almost the same code as
ktime_get_real_ts64() AFAICS, only simpler.

Plus, static vars need not be initialized to 0.

That could be dominated by the cost of ktime_get_real_ts64().
It also need synchronising instructions.
I did some testing. ktime_get_real_ts64() takes much less time than io
port access.

The test code is like:


The delta in 1 is about 500000ns. And delta in 2 is about
2000ns. The date is gotten on Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz.
So I suppose the impact of ktime_get_real_ts64 is small.

You may not be hitting the worst case for ktime_get_real_ts64(), though.

I wonder if special casing the virt guest would be a better approach.

Then, you could leave the code as is for non-virt and I'm not sure if the
delay is needed in the virt guest case at all.

So maybe do something like "if not in a virt guest, do the dummy inl()"
and that would be it?
After re-think the scenario again, I'd like to change the patch to
something like following as Rafael suggested:

If it's not in virt guest, we still use inl for dummy wait.
If it's in virt guest, we could assume inb will be trapped to HV and
remove the dummy wait.

I will generate v3 soon.

Yin, Fengwei