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