Re: [PATCH RFC hack dont apply] intel_idle: support running within a VM

Michael S. Tsirkin
2017-10-04T17:12:32Z

On Wed, Oct 04, 2017 at 10:09:39AM -0700, Jacob Pan wrote:
> On Wed, 4 Oct 2017 05:09:09 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> > On Mon, Oct 02, 2017 at 10:12:49AM -0700, Jacob Pan wrote:
> > > On Sat, 30 Sep 2017 01:21:43 +0200
> > > "Rafael J. Wysocki" <rafael@kernel.org> wrote:
> > >
> > > > On Sat, Sep 30, 2017 at 12:01 AM, Michael S. Tsirkin
> > > > <mst@redhat.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?
> >
> > Well we still want to e.g. mwait if possible - saves power.
> >
> > > 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
> >
> > It's not always a poll, on x86 putting the CPU in a low power state
> > is possible within a VM.
> >
> Are you talking about using mwait/monitor in the user space which are
> available on some Intel CPUs, such as Xeon Phi? I guess if the guest
> can identify host CPU id, it is doable.

Not really.

Please take a look at the patch in question - it does mwait in guest
kernel and no need to identify host CPU id.

> > Does not seem possible on other CPUs that's why it's vendor specific.
> >
>
> [Jacob Pan]