From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [PATCH 09/18] Robust TSC compensation Date: Tue, 13 Jul 2010 17:34:18 -0300 Message-ID: <20100713203418.GA903@amt.cnet> References: <1278987938-23873-1-git-send-email-zamsden@redhat.com> <1278987938-23873-10-git-send-email-zamsden@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: KVM , Avi Kivity , Glauber Costa , Linux-kernel To: Zachary Amsden Return-path: Content-Disposition: inline In-Reply-To: <1278987938-23873-10-git-send-email-zamsden@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Mon, Jul 12, 2010 at 04:25:29PM -1000, Zachary Amsden wrote: > Make the match of TSC find TSC writes that are close to each other > instead of perfectly identical; this allows the compensator to also > work in migration / suspend scenarios. > > Signed-off-by: Zachary Amsden > --- > arch/x86/kvm/x86.c | 14 ++++++++++---- > 1 files changed, 10 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 79c4608..51d3f3e 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -926,21 +926,27 @@ void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data) > struct kvm *kvm = vcpu->kvm; > u64 offset, ns, elapsed; > struct timespec ts; > + s64 sdiff; > > spin_lock(&kvm->arch.tsc_write_lock); > offset = data - native_read_tsc(); > ns = get_kernel_ns(); > elapsed = ns - kvm->arch.last_tsc_nsec; > + sdiff = data - kvm->arch.last_tsc_write; > + if (sdiff < 0) > + sdiff = -sdiff; > > /* > - * Special case: identical write to TSC within 5 seconds of > + * Special case: close write to TSC within 5 seconds of > * another CPU is interpreted as an attempt to synchronize > - * (the 5 seconds is to accomodate host load / swapping). > + * The 5 seconds is to accomodate host load / swapping as > + * well as any reset of TSC during the boot process. > * > * In that case, for a reliable TSC, we can match TSC offsets, > - * or make a best guest using kernel_ns value. > + * or make a best guest using elapsed value. > */ > - if (data == kvm->arch.last_tsc_write && elapsed < 5ULL * NSEC_PER_SEC) { > + if (sdiff < nsec_to_cycles(5ULL * NSEC_PER_SEC) && > + elapsed < 5ULL * NSEC_PER_SEC) { > if (!check_tsc_unstable()) { > offset = kvm->arch.last_tsc_offset; > pr_debug("kvm: matched tsc offset for %llu\n", data); What prevents a vcpu from seeing its TSC go backwards, in case the first write in the 5 second window is smaller than the victim vcpu's last visible TSC value ?