On 04/10/2017 20:31, Jacob Pan wrote: > On Wed, 4 Oct 2017 20:12:28 +0300 > "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> wrote: >>>>> >>>>>> On Sat, Sep 30, 2017 at 12:01 AM, Michael S. Tsirkin >>>>>> <mst@xxxxxxxxxx> 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. So I don't see how you can enter low > power state within VM guest. KVM does not exit on MWAIT (though it doesn't show it in CPUID by default), see commit 668fffa3f838edfcb1679f842f7ef1afa61c3e9a. Paolo > > +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] >