From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zachary Amsden Subject: Re: 2.6.35-rc1 regression with pvclock and smp guests Date: Mon, 02 Aug 2010 10:26:30 -1000 Message-ID: <4C5729F6.2050605@redhat.com> References: <4C4D4B8B.80006@amd.com> <4C4DDB00.50203@xutrox.com> <4C4F48D0.8090609@xutrox.com> <4C500872.1020809@redhat.com> <4C536F80.5090205@xutrox.com> <4C538CCE.1010104@redhat.com> <4C540EC9.1010008@xutrox.com> <4C54512B.6000307@xutrox.com> <4C54B7DE.4060901@redhat.com> <20100802144300.GD14448@mothafucka.localdomain> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="------------090404090004060504000803" Cc: Arjan Koers <0h61vkll2ly8@xutrox.com>, kvm@vger.kernel.org, Avi Kivity To: Glauber Costa Return-path: Received: from mx1.redhat.com ([209.132.183.28]:2942 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755223Ab0HBU0d (ORCPT ); Mon, 2 Aug 2010 16:26:33 -0400 In-Reply-To: <20100802144300.GD14448@mothafucka.localdomain> Sender: kvm-owner@vger.kernel.org List-ID: This is a multi-part message in MIME format. --------------090404090004060504000803 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit On 08/02/2010 04:43 AM, Glauber Costa wrote: > On Sat, Jul 31, 2010 at 01:55:10PM -1000, Zachary Amsden wrote: > >> On 07/31/2010 06:36 AM, Arjan Koers wrote: >> >>> On 2010-07-31 13:53, Arjan Koers wrote: >>> >>>> The kernel boots successfully when CONFIG_PRINTK_TIME is not set. >>>> >>>> >>> The problem occurs when this message is printed: >>> >>> [ 0.016000] kvm-clock: cpu 1, msr 0:1511c01, secondary cpu clock >>> >>> When I disable that printk, the kernel boots with >>> CONFIG_PRINTK_TIME=y >>> >>> --- a/arch/x86/kernel/kvmclock.c >>> +++ b/arch/x86/kernel/kvmclock.c >>> @@ -131,8 +131,8 @@ static int kvm_register_clock(char *txt) >>> int low, high; >>> low = (int)__pa(&per_cpu(hv_clock, cpu)) | 1; >>> high = ((u64)__pa(&per_cpu(hv_clock, cpu))>> 32); >>> - printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n", >>> - cpu, high, low, txt); >>> + /*printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n", >>> + cpu, high, low, txt);*/ >>> >>> return native_write_msr_safe(msr_kvm_system_time, low, high); >>> } >>> >>> So the problem appears to be that the clock of the second CPU >>> is used too soon (or that clock setup should finish earlier). >>> >> That's almost hilarious. The printk from setting up the kvm clock >> is invoking the kvm clock before it is setup. >> >> There's no reason other printks couldn't do the same thing, however. >> I think it's safest to keep an initialized flag and check for it >> before attempting to return a meaningful value. >> > I was on vacations, just got back. > > I think it is safe to just patch our own use of it. Before that, all other > printks will be handled by the main cpu anyway, since it'll be the only one active > at the moment. The only possible offenders for this are us, and the cpu initialization > code, which is already fragile in multiple ways anyway. > > A flag would only make things more complicated and dirty > Can we just do this? --------------090404090004060504000803 Content-Type: text/plain; name="zero.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="zero.patch" Initialize hv_clock to zero This stops callers from getting random values if data is accessed before clock is initialized; instead they will get zeroed clock values (because computation involves a multiplication by a factor in hv_clock). Signed-off-by: Zachary Amsden diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c index eb9b76c..e7acd0d 100644 --- a/arch/x86/kernel/kvmclock.c +++ b/arch/x86/kernel/kvmclock.c @@ -40,7 +40,7 @@ static int parse_no_kvmclock(char *arg) early_param("no-kvmclock", parse_no_kvmclock); /* The hypervisor will put information about time periodically here */ -static DEFINE_PER_CPU_SHARED_ALIGNED(struct pvclock_vcpu_time_info, hv_clock); +static DEFINE_PER_CPU_SHARED_ALIGNED(struct pvclock_vcpu_time_info, hv_clock) = {0}; static struct pvclock_wall_clock wall_clock; /* --------------090404090004060504000803--