From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756288AbbIXABY (ORCPT ); Wed, 23 Sep 2015 20:01:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48699 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756223AbbIXABX (ORCPT ); Wed, 23 Sep 2015 20:01:23 -0400 Date: Tue, 22 Sep 2015 17:23:47 -0300 From: Marcelo Tosatti To: Paolo Bonzini Cc: Radim =?utf-8?B?S3LEjW3DocWZ?= , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Luiz Capitulino Subject: Re: [PATCH 2/2] Revert "KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system MSR" Message-ID: <20150922202345.GA29417@amt.cnet> References: <1442591670-5216-1-git-send-email-rkrcmar@redhat.com> <1442591670-5216-3-git-send-email-rkrcmar@redhat.com> <20150922190133.GB23748@amt.cnet> <5601B191.4040809@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5601B191.4040809@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 Tue, Sep 22, 2015 at 09:52:49PM +0200, Paolo Bonzini wrote: > > > On 22/09/2015 21:01, Marcelo Tosatti wrote: > > On Fri, Sep 18, 2015 at 05:54:30PM +0200, Radim Krčmář wrote: > >> Shifting pvclock_vcpu_time_info.system_time on write to KVM system time > >> MSR is a change of ABI. Probably only 2.6.16 based SLES 10 breaks due > >> to its custom enhancements to kvmclock, but KVM never declared the MSR > >> only for one-shot initialization. (Doc says that only one write is > >> needed.) > >> > >> This reverts commit b7e60c5aedd2b63f16ef06fde4f81ca032211bc5. > >> And adds a note to the definition of PVCLOCK_COUNTS_FROM_ZERO. > >> > >> Signed-off-by: Radim Krčmář > >> --- > >> arch/x86/include/asm/pvclock-abi.h | 1 + > >> arch/x86/kvm/x86.c | 4 ---- > >> 2 files changed, 1 insertion(+), 4 deletions(-) > >> > >> diff --git a/arch/x86/include/asm/pvclock-abi.h b/arch/x86/include/asm/pvclock-abi.h > >> index 655e07a48f6c..67f08230103a 100644 > >> --- a/arch/x86/include/asm/pvclock-abi.h > >> +++ b/arch/x86/include/asm/pvclock-abi.h > >> @@ -41,6 +41,7 @@ struct pvclock_wall_clock { > >> > >> #define PVCLOCK_TSC_STABLE_BIT (1 << 0) > >> #define PVCLOCK_GUEST_STOPPED (1 << 1) > >> +/* PVCLOCK_COUNTS_FROM_ZERO broke ABI and can't be used anymore. */ > >> #define PVCLOCK_COUNTS_FROM_ZERO (1 << 2) > >> #endif /* __ASSEMBLY__ */ > >> #endif /* _ASM_X86_PVCLOCK_ABI_H */ > >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > >> index 18d59b584dee..34d33f4757d2 100644 > >> --- a/arch/x86/kvm/x86.c > >> +++ b/arch/x86/kvm/x86.c > >> @@ -1707,8 +1707,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) > >> vcpu->pvclock_set_guest_stopped_request = false; > >> } > >> > >> - pvclock_flags |= PVCLOCK_COUNTS_FROM_ZERO; > >> - > >> /* If the host uses TSC clocksource, then it is stable */ > >> if (use_master_clock) > >> pvclock_flags |= PVCLOCK_TSC_STABLE_BIT; > >> @@ -2006,8 +2004,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) > >> &vcpu->requests); > >> > >> ka->boot_vcpu_runs_old_kvmclock = tmp; > >> - > >> - ka->kvmclock_offset = -get_kernel_ns(); > >> } > >> > >> vcpu->arch.time = data; > >> -- > >> 2.5.2 > > > > ACK > > So I suppose you changed your mind :) but can you explain the reasoning? > > Paolo The patch is correct. Overflow (issue raised) is only an issue without PVCLOCK_TSC_STABLE_BIT.