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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	Yang Zhang <yang.zhang.wz@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	kvm@vger.kernel.org, Wanpeng Li <wanpeng.li@hotmail.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	rkrcmar@redhat.com, dmatlack@google.com, agraf@suse.de,
	Peter Zijlstra <peterz@infradead.org>,
	Len Brown <lenb@kernel.org>, Linux PM <linux-pm@vger.kernel.org>,
	"Raj, Ashok" <ashok.raj@intel.com>
Subject: Re: [PATCH RFC hack dont apply] intel_idle: support running within a VM
Date: Fri, 6 Oct 2017 06:37:18 +0300	[thread overview]
Message-ID: <20171006063433-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20171004113143.503032f0@jacob-builder>

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]