linux-kernel - Re: [PATCH RFC hack dont apply] intel

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Date:   Mon, 2 Oct 2017 10:12:49 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, rkrcmar@...hat.com,
        dmatlack@...gle.com, agraf@...e.de,
        Peter Zijlstra <peterz@...radead.org>,
        Len Brown <lenb@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH RFC hack dont apply] intel_idle: support running within
 a VM

On Sat, 30 Sep 2017 01:21:43 +0200
"Rafael J. Wysocki" <rafael@...nel.org> wrote:

> On Sat, Sep 30, 2017 at 12:01 AM, Michael S. Tsirkin <mst@...hat.com>
> wrote:
> > intel idle driver does not DTRT when running within a VM:
> > when going into a deep power state, the right thing to
> > do is to exit to hypervisor rather than to keep polling
> > within guest using mwait.
> >
> > Currently the solution is just to exit to hypervisor each time we go
> > idle - this is why kvm does not expose the mwait leaf to guests even
> > when it allows guests to do mwait.
> >
> > But that's not ideal - it seems better to use the idle driver to
> > guess when will the next interrupt arrive.  
> 
> The idle driver alone is not sufficient for that, though.
> 
I second that. Why try to solve this problem at vendor specific driver
level? perhaps just a pv idle driver that decide whether to vmexit
based on something like local per vCPU timer expiration? I guess we
can't predict other wake events such as interrupts.
e.g.
if (get_next_timer_interrupt() > kvm_halt_target_residency)
	vmexit
else
	poll

Jacob