Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755994Ab0FOHfH (ORCPT ); Tue, 15 Jun 2010 03:35:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63330 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755716Ab0FOHfB (ORCPT ); Tue, 15 Jun 2010 03:35:01 -0400 From: Zachary Amsden To: avi@redhat.com, mtosatti@redhat.com, glommer@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Zachary Amsden Subject: [PATCH 16/17] TSC reset compensation Date: Mon, 14 Jun 2010 21:34:18 -1000 Message-Id: <1276587259-32319-17-git-send-email-zamsden@redhat.com> In-Reply-To: <1276587259-32319-1-git-send-email-zamsden@redhat.com> References: <1276587259-32319-1-git-send-email-zamsden@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2340 Lines: 69 Attempt to synchronize TSCs which are reset to the same value. In the case of a reliable hardware TSC, we can just re-use the same offset, but on non-reliable hardware, we can get closer by adjusting the offset to match the elapsed time. Signed-off-by: Zachary Amsden --- arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++-- 1 files changed, 32 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 8e836e9..cedb71f 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v) set_bit(KVM_REQ_CLOCK_SYNC, &v->requests); } +static inline int kvm_tsc_reliable(void) +{ + return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) && + !check_tsc_unstable()); +} + void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data) { struct kvm *kvm = vcpu->kvm; - u64 offset; + u64 offset, ns, elapsed; spin_lock(&kvm->arch.tsc_write_lock); offset = data - native_read_tsc(); - kvm->arch.last_tsc_nsec = get_kernel_ns(); + ns = get_kernel_ns(); + elapsed = ns - kvm->arch.last_tsc_nsec; + + /* + * Special case: identical 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). + * + * In that case, for a reliable TSC, we can match TSC offsets, + * or make a best guest using kernel_ns value. + */ + if (data == kvm->arch.last_tsc_write && elapsed < 5 * NSEC_PER_SEC) { + if (kvm_tsc_reliable()) { + offset = kvm->arch.last_tsc_offset; + pr_debug("kvm: matched tsc offset for %llu\n", data); + } else { + u64 tsc_delta = elapsed * __get_cpu_var(cpu_tsc_khz); + tsc_delta = tsc_delta / USEC_PER_SEC; + offset -= tsc_delta; + pr_debug("kvm: adjusted tsc offset by %llu\n", tsc_delta); + } + ns = kvm->arch.last_tsc_nsec; + } + kvm->arch.last_tsc_nsec = ns; kvm->arch.last_tsc_write = data; kvm->arch.last_tsc_offset = offset; kvm_x86_ops->write_tsc_offset(vcpu, offset); -- 1.7.1 -- 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/