Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932226Ab2BOLXb (ORCPT ); Wed, 15 Feb 2012 06:23:31 -0500 Received: from mx1.redhat.com ([209.132.183.28]:23678 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757300Ab2BOLXa (ORCPT ); Wed, 15 Feb 2012 06:23:30 -0500 Message-ID: <4F3B95A5.3050704@redhat.com> Date: Wed, 15 Feb 2012 12:23:17 +0100 From: Igor Mammedov User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0) Gecko/20120131 Thunderbird/10.0 MIME-Version: 1.0 To: Avi Kivity CC: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, riel@redhat.com, amit.shah@redhat.com, mtosatti@redhat.com Subject: Re: [PATCH] BUG in pv_clock when overflow condition is detected References: <1329157256-19926-1-git-send-email-imammedo@redhat.com> <4F3B8DC9.8040100@redhat.com> In-Reply-To: <4F3B8DC9.8040100@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2009 Lines: 53 On 02/15/2012 11:49 AM, Avi Kivity wrote: > On 02/13/2012 08:20 PM, Igor Mammedov wrote: >> BUG when overflow occurs at pvclock.c:pvclock_get_nsec_offset >> >> u64 delta = native_read_tsc() - shadow->tsc_timestamp; >> >> this might happen at an attempt to read an uninitialized yet clock. >> It won't prevent stalls and hangs but at least it won't do it silently. >> >> Signed-off-by: Igor Mammedov >> --- >> arch/x86/kernel/pvclock.c | 5 ++++- >> 1 files changed, 4 insertions(+), 1 deletions(-) >> >> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c >> index 42eb330..35a6190 100644 >> --- a/arch/x86/kernel/pvclock.c >> +++ b/arch/x86/kernel/pvclock.c >> @@ -43,7 +43,10 @@ void pvclock_set_flags(u8 flags) >> >> static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow) >> { >> - u64 delta = native_read_tsc() - shadow->tsc_timestamp; >> + u64 delta; >> + u64 tsc = native_read_tsc(); >> + BUG_ON(tsc< shadow->tsc_timestamp); >> + delta = tsc - shadow->tsc_timestamp; >> return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul, >> shadow->tsc_shift); > > Maybe a WARN_ON_ONCE()? Otherwise a relatively minor hypervisor bug can > kill the guest. An attempt to print from this place is not perfect since it often leads to recursive calling to this very function and it hang there anyway. But if you insist I'll re-post it with WARN_ON_ONCE, It won't make much difference because guest will hang/stall due overflow anyway. If there is an intention to keep guest functional after the event then maybe this patch is a way to go http://www.spinics.net/lists/kvm/msg68463.html this way clock will be re-silent to this kind of errors, like bare-metal one is. -- Thanks, Igor -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/