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

Paolo Bonzini
2017-10-05T10:45:19Z

On 04/10/2017 20:31, 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. 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]
>