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 2AA95C10F0E for ; Mon, 15 Apr 2019 16:32:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 035B02183F for ; Mon, 15 Apr 2019 16:32:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727490AbfDOQcw (ORCPT ); Mon, 15 Apr 2019 12:32:52 -0400 Received: from mga17.intel.com ([192.55.52.151]:5769 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726094AbfDOQcw (ORCPT ); Mon, 15 Apr 2019 12:32:52 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 15 Apr 2019 09:32:52 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,354,1549958400"; d="scan'208";a="337796397" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.181]) by fmsmga005.fm.intel.com with ESMTP; 15 Apr 2019 09:32:51 -0700 Date: Mon, 15 Apr 2019 09:32:51 -0700 From: Sean Christopherson To: Liran Alon Cc: Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , kvm@vger.kernel.org, Wanpeng Li Subject: Re: [PATCH 6/7] KVM: lapic: Clean up the code for handling of a pre-expired hv_timer Message-ID: <20190415163251.GE24010@linux.intel.com> References: <20190412201834.10831-1-sean.j.christopherson@intel.com> <20190412201834.10831-7-sean.j.christopherson@intel.com> <9DCCF040-F617-4E10-9AA3-715F6EB51ADA@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <9DCCF040-F617-4E10-9AA3-715F6EB51ADA@oracle.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 Sun, Apr 14, 2019 at 03:15:41PM +0300, Liran Alon wrote: > > > > On 12 Apr 2019, at 23:18, Sean Christopherson wrote: > > > > Calling apic_timer_expired() is a nop when a timer interrupt is already > > pending, i.e. there's no need to call apic_timer_expired() when there's > > a pending interrupt and the hv_timer wants to pend its own interrupt. > > Separate the two flows to make the code more readable and to avoid an > > unnecessary function call and read to ktimer->pending. > > In case timer is not periodic and r==1, atomic_read(&ktimer->pending) is not executed. > > > > > Cc: Wanpeng Li > > Signed-off-by: Sean Christopherson > > --- > > arch/x86/kvm/lapic.c | 9 ++++++--- > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > > index 1d649a2af04c..f0be6f148a47 100644 > > --- a/arch/x86/kvm/lapic.c > > +++ b/arch/x86/kvm/lapic.c > > @@ -1703,9 +1703,12 @@ static bool start_hv_timer(struct kvm_lapic *apic) > > * the window. For periodic timer, leave the hv timer running for > > * simplicity, and the deadline will be recomputed on the next vmexit. > > */ > > - if (!apic_lvtt_period(apic) && (r || atomic_read(&ktimer->pending))) { > > - if (r) > > - apic_timer_expired(apic); > > + if (!apic_lvtt_period(apic) && atomic_read(&ktimer->pending)) > > + return false; > > + > > + /* set_hv_timer() returns '1' when the timer has already expired. */ > > + if (r) { > > + apic_timer_expired(apic); > > return false; > > } > > > > -- > > 2.21.0 > > > > First, I think you should emphasise in commit message that you have actually > fixed a rare bug here. In case timer is periodic but given > ktimer->tscdeadline has already expired on host, we should call > apic_timer_expired(). Heh, I actually didn't even catch that bug, I was simply cleaning up the code because I had a hard time following the logic. > In addition, when start_hv_timer() returns false, restart_apic_timer() just > calls start_sw_timer() which use hrtimer instead of VMX preemption timer. > Therefore, it seems a bit ineffective to me for start_hv_timer() to return > false in case ktimer->pending or when ktimer->tscdeadline already expired. > Shouldn’t we return true in these cases? That also seemed weird to me. Again, I had a hell of a time following the intended logic and didn't want to break anything. AFAICT, the motivation for calling start_sw_timer() is to cancel the HV timer, and possibly to ensure start_sw_period() is called when necessary. But the latter will be handled by virtue of checking "r" after apic_lvtt_period(), so this? if (r) { apic_timer_expired(apic); ktimer->hv_timer_in_use = false; return true; }