From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754518Ab2A3QTX (ORCPT ); Mon, 30 Jan 2012 11:19:23 -0500 Received: from david.siemens.de ([192.35.17.14]:18862 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754492Ab2A3QTT (ORCPT ); Mon, 30 Jan 2012 11:19:19 -0500 Message-ID: <4F26C2EC.5070303@siemens.com> Date: Mon, 30 Jan 2012 17:18:52 +0100 From: Jan Kiszka User-Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 MIME-Version: 1.0 To: Avi Kivity CC: Eric B Munson , mingo@redhat.com, hpa@zytor.com, ryanh@linux.vnet.ibm.com, aliguori@us.ibm.com, mtosatti@redhat.com, jeremy.fitzhardinge@citrix.com, kvm@vger.kernel.org, linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST_STOPPED References: <1326825641-15765-1-git-send-email-emunson@mgebm.net> <1326825641-15765-4-git-send-email-emunson@mgebm.net> <4F26B220.9050101@redhat.com> In-Reply-To: <4F26B220.9050101@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2012-01-30 16:07, Avi Kivity wrote: > On 01/17/2012 08:40 PM, Eric B Munson wrote: >> Now that we have a flag that will tell the guest it was suspended, create an >> interface for that communication using a KVM ioctl. >> >> >> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt >> index e1d94bf..1931e5c 100644 >> --- a/Documentation/virtual/kvm/api.txt >> +++ b/Documentation/virtual/kvm/api.txt >> @@ -1491,6 +1491,19 @@ following algorithm: >> Some guests configure the LINT1 NMI input to cause a panic, aiding in >> debugging. >> >> +4.65 KVMCLOCK_GUEST_PAUSED >> + >> +Capability: KVM_CAP_GUEST_PAUSED >> +Architechtures: Any that implement pvclocks (currently x86 only) >> +Type: vcpu ioctl > > vm ioctl. > >> +Parameters: None >> +Returns: 0 on success, -1 on error >> + >> +This signals to the host kernel that the specified guest is being paused by >> +userspace. The host will set a flag in the pvclock structure that is checked >> +from the soft lockup watchdog. This ioctl can be called during pause or >> +unpause. >> + >> 5. The kvm_run structure >> >> >> +/* >> + * kvm_set_guest_paused() indicates to the guest kernel that it has been >> + * stopped by the hypervisor. This function will be called from the host only. >> + */ >> +static int kvm_set_guest_paused(struct kvm *kvm) >> +{ >> + struct kvm_vcpu *vcpu; >> + struct pvclock_vcpu_time_info *src; >> + int i; >> + >> + kvm_for_each_vcpu(i, vcpu, kvm) { >> + if (!vcpu->arch.time_page) >> + continue; >> + src = &vcpu->arch.hv_clock; >> + src->flags |= PVCLOCK_GUEST_STOPPED; > > This looks racy. The vcpu can remove its kvmclock concurrently with > this access, and src will be NULL. There is no race here (src is member of the vcpu), but arch.time might have become invalid. KVM_REQ_CLOCK_UPDATE instead of mark_page_dirty would indeed be the way to go. Trivial solution, I would say. However, the concept of "guest stopped" has VM, not VCPU scope. That makes the call more appropriate as a VM ioctl. If that thing should really become per-vcpu, at least call it KVMCLOCK_VCPU_STOPPED. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux