linux-kernel - Re: [RESEND] ACPI / processor_idle: use dead loop instead of io port access for wait

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Date:   Mon, 14 Oct 2019 11:38:13 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     Len Brown <lenb@...nel.org>,
        "open list:ACPI" <linux-acpi@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND] ACPI / processor_idle: use dead loop instead of io port access for wait

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. :-)