From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 65C08C282DA for ; Wed, 17 Apr 2019 14:34:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3B9BE206BA for ; Wed, 17 Apr 2019 14:34:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731745AbfDQOe1 (ORCPT ); Wed, 17 Apr 2019 10:34:27 -0400 Received: from mga01.intel.com ([192.55.52.88]:24134 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731454AbfDQOe1 (ORCPT ); Wed, 17 Apr 2019 10:34:27 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 17 Apr 2019 07:34:25 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,362,1549958400"; d="scan'208";a="292335849" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.181]) by orsmga004.jf.intel.com with ESMTP; 17 Apr 2019 07:34:26 -0700 Date: Wed, 17 Apr 2019 07:34:26 -0700 From: Sean Christopherson To: Paolo Bonzini Cc: Radim =?utf-8?B?S3LEjW3DocWZ?= , kvm@vger.kernel.org, Liran Alon , Wanpeng Li Subject: Re: [PATCH v3 1/9] KVM: lapic: Hard cap the auto-calculated timer advancement Message-ID: <20190417143426.GB8567@linux.intel.com> References: <20190416203248.29429-1-sean.j.christopherson@intel.com> <20190416203248.29429-2-sean.j.christopherson@intel.com> <62b9be92-303f-d740-180f-7c7bbc95c98d@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <62b9be92-303f-d740-180f-7c7bbc95c98d@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Apr 17, 2019 at 02:57:55PM +0200, Paolo Bonzini wrote: > On 16/04/19 22:32, Sean Christopherson wrote: > > To minimize the latency of timer interrupts as observed by the guest, > > KVM adjusts the values it programs into the host timers to account for > > the host's overhead of programming and handling the timer event. Now > > that the timer advancement is automatically tuned during runtime, it's > > effectively unbounded by default, e.g. if KVM is running as L1 the > > advancement can measure in hundreds of milliseconds. > > > > Place a somewhat arbitrary hard cap of 5000ns on the auto-calculated > > advancement, as large advancements can break reasonable assumptions of > > the guest, e.g. that a timer configured to fire after 1ms won't arrive > > on the next instruction. Although KVM busy waits to mitigate the timer > > event arriving too early, complications can arise when shifting the > > interrupt too far, e.g. vmx.flat/interrupt in kvm-unit-tests will fail > > when its "host" exits on interrupts (because the INTR is injected before > > the gets executes STI+HLT). Arguably the unit test is "broken" in the > > sense that delaying the timer interrupt by 1ms doesn't technically > > guarantee the interrupt will arrive after STI+HLT, but it's a reasonable > > assumption that KVM should support. > > > > Furthermore, an unbounded advancement also effectively unbounds the time > > spent busy waiting, e.g. if the guest programs a timer with a very large > > delay. > > > > Arguably the advancement logic could simply be disabled when running as > > L1, but KVM needs to bound the advancement time regardless, e.g. if the > > TSC is unstable and the calculations get wildly out of whack. And > > allowing the advancement when running as L1 is a good stress test of > > sorts for the logic. > > > > Cc: Liran Alon > > Cc: Wanpeng Li > > Fixes: 3b8a5df6c4dc6 ("KVM: LAPIC: Tune lapic_timer_advance_ns automatically") > > Signed-off-by: Sean Christopherson > > --- > > arch/x86/kvm/lapic.c | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > > index 9bf70cf84564..92446cba9b24 100644 > > --- a/arch/x86/kvm/lapic.c > > +++ b/arch/x86/kvm/lapic.c > > @@ -74,6 +74,7 @@ static bool lapic_timer_advance_adjust_done = false; > > #define LAPIC_TIMER_ADVANCE_ADJUST_DONE 100 > > /* step-by-step approximation to mitigate fluctuation */ > > #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8 > > +#define LAPIC_TIMER_ADVANCE_MAX_NS 5000 > > > > static inline int apic_test_vector(int vec, void *bitmap) > > { > > @@ -1522,6 +1523,10 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu) > > } > > if (abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE) > > lapic_timer_advance_adjust_done = true; > > + if (unlikely(lapic_timer_advance_ns > LAPIC_TIMER_ADVANCE_MAX_NS)) { > > + lapic_timer_advance_ns = LAPIC_TIMER_ADVANCE_MAX_NS; > > + lapic_timer_advance_adjust_done = true; > > + } > > I would treat this case as "advancing the timer has failed miserably" > and reset lapic_timer_advance_ns to 0. As in, disable the feature, correct? That works for me.