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From:   "Rafael J. Wysocki" <rjw@rjwysocki.net>
To:     "Yin, Fengwei" <fengwei.yin@intel.com>
Cc:     Len Brown <lenb@kernel.org>,
        "open list:ACPI" <linux-acpi@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [RESEND] ACPI / processor_idle: use dead loop instead of io port access for wait
Date:   Mon, 14 Oct 2019 11:38:13 +0200
Message-ID: <3727681.FYoUZqeJdN@kreacher>
In-Reply-To: <8a65bf97-c066-8e5e-ba82-75e2a6fd5b45@intel.com>
References: <20190909073937.31554-1-fengwei.yin@intel.com> <12278756.3dKznOqol2@kreacher> <8a65bf97-c066-8e5e-ba82-75e2a6fd5b45@intel.com>
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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?

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