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Subject: Re: [RESEND] ACPI / processor_idle: use dead loop instead of io port
 access for wait
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     Len Brown <lenb@kernel.org>,
        "open list:ACPI" <linux-acpi@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
References: <20190909073937.31554-1-fengwei.yin@intel.com>
 <12278756.3dKznOqol2@kreacher>
 <8a65bf97-c066-8e5e-ba82-75e2a6fd5b45@intel.com>
 <3727681.FYoUZqeJdN@kreacher>
From:   "Yin, Fengwei" <fengwei.yin@intel.com>
Message-ID: <187d70f6-3c41-a42f-26f1-9f3317ccb7da@intel.com>
Date:   Tue, 15 Oct 2019 16:03:13 +0800
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On 10/14/2019 5:38 PM, Rafael J. Wysocki wrote:
> On Friday, October 11, 2019 3:30:41 PM CEST Yin, Fengwei wrote:
>>
>> On 10/11/2019 5:05 PM, Rafael J. Wysocki wrote:
>>> Sorry for the delay.
>> No problem.
>>
>>>
>>> On Monday, September 9, 2019 9:39:37 AM CEST Yin Fengwei wrote:
>>>> In function acpi_idle_do_entry(), we do an io port access to guarantee
>>>> hardware behavior. But it could trigger unnecessary vmexit for
>>>> virtualization environemnt.
>>>
>>> Is this a theoretical problem, or do you actually see it?
>>>
>>> If you see it, I'd like to have a pointer to a bug report regarding it
>>> or similar.
>> We did see this issue when we run linux as guest with ACRN hypervisor
>> instead of kvm or xen. In our case, we export all native C states to
>> guest and let guest choose which C state it will enter.
>>
>> And we observed many pm timer port access when guest tried to enter
>> deeper C state (Yes, we emulate pm timer so pm timer access will trigger
>> vmexit).
> 
> Can you please put this information into the changelog of your patch?
I added this information to the patch commit message and sent out v2.
Thanks a lot for reviewing and comments.

Regards
Yin, Fengwei

> 
> It works very well as a rationale for me. :-)
> 
> 
>