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

Michael S. Tsirkin
2017-10-04T02:09:15Z

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.

Does not seem possible on other CPUs that's why it's vendor specific.

--
MST