From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753967AbdF0WKs (ORCPT ); Tue, 27 Jun 2017 18:10:48 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:33937 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753605AbdF0WKc (ORCPT ); Tue, 27 Jun 2017 18:10:32 -0400 MIME-Version: 1.0 In-Reply-To: References: <1498219190-6457-1-git-send-email-wanpeng.li@hotmail.com> From: Wanpeng Li Date: Wed, 28 Jun 2017 06:10:30 +0800 Message-ID: Subject: Re: [PATCH v2] KVM: LAPIC: Fix lapic timer injection delay To: Paolo Bonzini Cc: "linux-kernel@vger.kernel.org" , kvm , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2017-06-28 6:09 GMT+08:00 Wanpeng Li : > 2017-06-28 0:10 GMT+08:00 Paolo Bonzini : >> >> >> On 23/06/2017 13:59, Wanpeng Li wrote: >>> From: Wanpeng Li >>> >>> Linux as a guest will prevent to program the next event to the clock event >>> device when the event is in the past. However, it is not guaranteed by all >>> the guests, the guest like kvm-unit-tests/apic.flat will write current tsc >>> value visible in guest to MSR_IA32_TSCDEADLINE, lapic timer which is emulated >>> by vmx preemption timer will program the absolute target tsc value to vmcs >>> preemption timer field w/ delta == 0, then plays a vmentry and an upcoming >>> vmx preemption timer fire vmexit dance, the lapic timer injection is delayed >>> for this duration. Actually the lapic timer which is emulated by hrtimer can >>> handle this correctly. >>> >>> This patch fixes it by firing the lapic timer and injecting a timer interrupt >>> immediately during the next vmentry if guest programs an expired timer to >>> the emulated timer device. This saves ~300 cycles on the tsc_deadline_timer >>> test of apic.flat. >> >> If it doesn't matter for actual guests, I'd rather not complicate the logic. > > Sorry, maybe my description is not that accurate. It can also benefit > if the tsc deadline timer is programmed really close to the deadline. > That's why we introduce commit 9175d2e97b ("KVM: vmx: fix underflow in > TSC deadline calculation"). In addition, I capture there is frequently > programmed close to the deadline in vmx_set_hv_timer(delta_tsc == 0): I test a linux guest. > > qemu-system-x86-2352 [003] .... 457.709435: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [004] .... 457.941090: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.381340: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.381565: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.382079: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.382277: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.382576: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.383065: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.565506: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.565718: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.566199: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.566431: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.566680: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [005] .... 458.567154: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [006] .... 458.989358: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [006] .... 458.989554: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [006] .... 458.990075: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.397456: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.397618: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.397877: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.398091: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.398307: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.398672: vmx_set_hv_timer: expired timer > qemu-system-x86-2352 [003] .... 459.502826: vmx_set_hv_timer: expired timer > > Regards, > Wanpeng Li