On Wed, 4 Oct 2017 20:12:28 +0300
"Michael S. Tsirkin" [off-list ref] wrote:

On Wed, Oct 04, 2017 at 10:09:39AM -0700, Jacob Pan wrote:

quoted

On Wed, 4 Oct 2017 05:09:09 +0300
"Michael S. Tsirkin" [off-list ref] wrote:
  

quoted

On Mon, Oct 02, 2017 at 10:12:49AM -0700, Jacob Pan wrote:  

quoted

On Sat, 30 Sep 2017 01:21:43 +0200
"Rafael J. Wysocki" [off-list ref] wrote:
    

quoted

On Sat, Sep 30, 2017 at 12:01 AM, Michael S. Tsirkin
[off-list ref] wrote:    

quoted

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.
  

quoted

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.

+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;
+}

quoted

quoted

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

[Jacob Pan]  

[Jacob Pan]