From mboxrd@z Thu Jan 1 00:00:00 1970 From: Denis Plotnikov Subject: [PATCH 2/2] KVM: x86: fix maintaining of kvm_clock stability on guest CPU hotplug Date: Thu, 6 Apr 2017 11:08:45 +0300 Message-ID: <1491466125-16988-3-git-send-email-dplotnikov@virtuozzo.com> References: <1491466125-16988-1-git-send-email-dplotnikov@virtuozzo.com> Mime-Version: 1.0 Content-Type: text/plain Cc: , , To: , , , Return-path: Received: from mail-he1eur01on0110.outbound.protection.outlook.com ([104.47.0.110]:59105 "EHLO EUR01-HE1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753683AbdDFIJT (ORCPT ); Thu, 6 Apr 2017 04:09:19 -0400 In-Reply-To: <1491466125-16988-1-git-send-email-dplotnikov@virtuozzo.com> Sender: kvm-owner@vger.kernel.org List-ID: VCPU TSC synchronization is perfromed in kvm_write_tsc() when the TSC value being set is within 1 second from the expected, as obtained by extrapolating of the TSC in already synchronized VCPUs. This is naturally achieved on all VCPUs at VM start and resume; however on VCPU hotplug it is not: the newly added VCPU is created with TSC == 0 while others are well ahead. To compensate for that, consider host-initiated kvm_write_tsc() with TSC == 0 a special case requiring synchronization regardless of the current TSC on other VCPUs. Signed-off-by: Denis Plotnikov Reviewed-by: Roman Kagan --- arch/x86/kvm/x86.c | 26 +++++++++++++++++--------- 1 file changed, 17 insertions(+), 9 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 4c674eb..a4091ee 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1455,15 +1455,23 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr) elapsed = ns - kvm->arch.last_tsc_nsec; if (vcpu->arch.virtual_tsc_khz) { - u64 tsc_exp = kvm->arch.last_tsc_write + - nsec_to_cycles(vcpu, elapsed); - u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL; - /* - * Special case: TSC write with a small delta (1 second) of virtual - * cycle time against real time is interpreted as an attempt to - * synchronize the CPU. - */ - synchronizing = data < tsc_exp + tsc_hz && data > tsc_exp - tsc_hz; + if ((data == 0) && msr->host_initiated) { + /* + * detection of vcpu initialization -- need to sync with other vCPUs + * particularly helps to keep kvm_clock stable after CPU hotplug + */ + synchronizing = true; + } else { + u64 tsc_exp = kvm->arch.last_tsc_write + + nsec_to_cycles(vcpu, elapsed); + u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL; + /* + * Special case: TSC write with a small delta (1 second) of virtual + * cycle time against real time is interpreted as an attempt to + * synchronize the CPU. + */ + synchronizing = data < tsc_exp + tsc_hz && data > tsc_exp - tsc_hz; + } } /* -- 1.8.3.1