From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dor Laor Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host Date: Sun, 11 Dec 2011 14:40:58 +0200 Message-ID: <4EE4A4DA.90100@redhat.com> References: <1323116344-17911-1-git-send-email-emunson@mgebm.net> <4EDF7B0D.4060001@redhat.com> Reply-To: dlaor@redhat.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Eric B Munson , mingo@redhat.com, hpa@zytor.com, arnd@arndb.de, ryanh@linux.vnet.ibm.com, aliguori@us.ibm.com, mtosatti@redhat.com, jeremy.fitzhardinge@citrix.com, levinsasha928@gmail.com, Jan Kiszka , kvm@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org To: Avi Kivity Return-path: In-Reply-To: <4EDF7B0D.4060001@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 12/07/2011 04:41 PM, Avi Kivity wrote: > On 12/05/2011 10:18 PM, Eric B Munson wrote: >> Changes from V4: >> Rename KVM_GUEST_PAUSED to KVMCLOCK_GUEST_PAUSED >> Add description of KVMCLOCK_GUEST_PAUSED ioctl to api.txt >> >> Changes from V3: >> Include CC's on patch 3 >> Drop clear flag ioctl and have the watchdog clear the flag when it is reset >> >> Changes from V2: >> A new kvm functions defined in kvm_para.h, the only change to pvclock is the >> initial flag definition >> >> Changes from V1: >> (Thanks Marcelo) >> Host code has all been moved to arch/x86/kvm/x86.c >> KVM_PAUSE_GUEST was renamed to KVM_GUEST_PAUSED >> >> When a guest kernel is stopped by the host hypervisor it can look like a soft >> lockup to the guest kernel. This false warning can mask later soft lockup >> warnings which may be real. This patch series adds a method for a host >> hypervisor to communicate to a guest kernel that it is being stopped. The >> final patch in the series has the watchdog check this flag when it goes to >> issue a soft lockup warning and skip the warning if the guest knows it was >> stopped. >> >> It was attempted to solve this in Qemu, but the side effects of saving and >> restoring the clock and tsc for each vcpu put the wall clock of the guest behind >> by the amount of time of the pause. This forces a guest to have ntp running >> in order to keep the wall clock accurate. Guests need to run NTP regardless, not only the virtualization layer add some skew, the physical world is not that perfect. btw: traditional NTP client won't sync the time automatically if the diff is > 0.5%. > > Having this controlled from userspace means it doesn't work for SIGSTOP > or for long scheduling delays. What about doing this automatically > based on preempt notifiers? > > Isn't it solved by steal time?