From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH RFC hack dont apply] intel_idle: support running within a VM Date: Fri, 6 Oct 2017 06:37:18 +0300 Message-ID: <20171006063433-mutt-send-email-mst@kernel.org> References: <20170930005046-mutt-send-email-mst@kernel.org> <20171002101249.69b5611a@jacob-builder> <20171004050706-mutt-send-email-mst@kernel.org> <20171004100939.1893e4a7@jacob-builder> <20171004201125-mutt-send-email-mst@kernel.org> <20171004113143.503032f0@jacob-builder> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Rafael J. Wysocki" , Yang Zhang , Linux Kernel Mailing List , kvm@vger.kernel.org, Wanpeng Li , Paolo Bonzini , Thomas Gleixner , rkrcmar@redhat.com, dmatlack@google.com, agraf@suse.de, Peter Zijlstra , Len Brown , Linux PM , "Raj, Ashok" To: Jacob Pan Return-path: Content-Disposition: inline In-Reply-To: <20171004113143.503032f0@jacob-builder> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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" 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" 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" wrote: > > > > > > > > > > > On Sat, Sep 30, 2017 at 12:01 AM, Michael S. Tsirkin > > > > > > 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]