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SubjectRe: [PATCH RFC hack dont apply] intel_idle: support running within a VM
On Wed, Oct 04, 2017 at 11:31:43AM -0700, Jacob Pan wrote:
> On Wed, 4 Oct 2017 20:12:28 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> > 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.
> >
> I may be missing something, in your patch I only see HLT being used in
> the guest OS, that would cause VM exit right? If you do mwait in the
> guest kernel, it will also exit.


No mwait won't exit if running on kvm.
See 668fffa3f838edfcb1679f842f7ef1afa61c3e9a


> So I don't see how you can enter low
> power state within VM guest.
>
> +static int intel_halt(struct cpuidle_device *dev,
> + struct cpuidle_driver *drv, int index)
> +{
> + printk_once(KERN_ERR "safe_halt started\n");
> + safe_halt();
> + printk_once(KERN_ERR "safe_halt done\n");
> + return index;
> +}
> >
> > > > Does not seem possible on other CPUs that's why it's vendor
> > > > specific.
> > >
> > > [Jacob Pan]
>
> [Jacob Pan]

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