From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757067Ab1LNMTx (ORCPT ); Wed, 14 Dec 2011 07:19:53 -0500 Received: from mx1.redhat.com ([209.132.183.28]:62862 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755664Ab1LNMTn (ORCPT ); Wed, 14 Dec 2011 07:19:43 -0500 Date: Wed, 14 Dec 2011 10:16:22 -0200 From: Marcelo Tosatti To: Avi Kivity Cc: Eric B Munson , mingo@redhat.com, hpa@zytor.com, arnd@arndb.de, ryanh@linux.vnet.ibm.com, aliguori@us.ibm.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 Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host Message-ID: <20111214121622.GB18317@amt.cnet> References: <1323116344-17911-1-git-send-email-emunson@mgebm.net> <4EDF7B0D.4060001@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4EDF7B0D.4060001@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 07, 2011 at 04:41:17PM +0200, 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. > > 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? Long scheduling delays should be considered hangups from the guest perspective. About SIGSTOP, that is a corner case. Unsure if its even properly supported by QEMU.