Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757295AbbDVNeL (ORCPT ); Wed, 22 Apr 2015 09:34:11 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:35395 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756819AbbDVNeJ (ORCPT ); Wed, 22 Apr 2015 09:34:09 -0400 Date: Wed, 22 Apr 2015 14:34:01 +0100 From: Luis Henriques To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, saso.linux@astim.si, lists2009@fnarfbargle.com, Nadav Amit , stable@vger.kernel.org Subject: Re: [PATCH stable] KVM: x86: Fix lost interrupt on irr_pending race Message-ID: <20150422133401.GE3671@charon> References: <1429602745-29882-1-git-send-email-pbonzini@redhat.com> <55360EA9.2030806@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <55360EA9.2030806@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3738 Lines: 100 On Tue, Apr 21, 2015 at 10:47:37AM +0200, Paolo Bonzini wrote: > > > On 21/04/2015 09:52, Paolo Bonzini wrote: > > From: Nadav Amit > > > > [ upstream commit f210f7572bedf3320599e8b2d8e8ec2d96270d0b ] > > > > apic_find_highest_irr assumes irr_pending is set if any vector in APIC_IRR is > > set. If this assumption is broken and apicv is disabled, the injection of > > interrupts may be deferred until another interrupt is delivered to the guest. > > Ultimately, if no other interrupt should be injected to that vCPU, the pending > > interrupt may be lost. > > > > commit 56cc2406d68c ("KVM: nVMX: fix "acknowledge interrupt on exit" when APICv > > is in use") changed the behavior of apic_clear_irr so irr_pending is cleared > > after setting APIC_IRR vector. After this commit, if apic_set_irr and > > apic_clear_irr run simultaneously, a race may occur, resulting in APIC_IRR > > vector set, and irr_pending cleared. In the following example, assume a single > > vector is set in IRR prior to calling apic_clear_irr: > > > > apic_set_irr apic_clear_irr > > ------------ -------------- > > apic->irr_pending = true; > > apic_clear_vector(...); > > vec = apic_search_irr(apic); > > // => vec == -1 > > apic_set_vector(...); > > apic->irr_pending = (vec != -1); > > // => apic->irr_pending == false > > > > Nonetheless, it appears the race might even occur prior to this commit: > > > > apic_set_irr apic_clear_irr > > ------------ -------------- > > apic->irr_pending = true; > > apic->irr_pending = false; > > apic_clear_vector(...); > > if (apic_search_irr(apic) != -1) > > apic->irr_pending = true; > > // => apic->irr_pending == false > > apic_set_vector(...); > > > > Fixing this issue by: > > 1. Restoring the previous behavior of apic_clear_irr: clear irr_pending, call > > apic_clear_vector, and then if APIC_IRR is non-zero, set irr_pending. > > 2. On apic_set_irr: first call apic_set_vector, then set irr_pending. > > > > Signed-off-by: Nadav Amit > > Fixes: 33e4c68656a2e461b296ce714ec322978de85412 > > Cc: stable@vger.kernel.org # 2.6.32+ > > Signed-off-by: Paolo Bonzini > > --- > > The race was reported in 3.17+ by Brad Campbell and in > > 2.6.32 by Saso Slavicic, so it qualifies for stable. > > Patch for kernels before 3.17: > Thanks Paolo. I was going to apply this backport to the 3.16 kernel but it looks like the original commit is a clean cherry-pick. Shall I still apply your backport, or do you think the original commit should be applied instead? Cheers, -- Lu?s > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index 6e8ce5a1a05d..e0e5642dae41 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -341,8 +341,12 @@ EXPORT_SYMBOL_GPL(kvm_apic_update_irr); > > static inline void apic_set_irr(int vec, struct kvm_lapic *apic) > { > - apic->irr_pending = true; > apic_set_vector(vec, apic->regs + APIC_IRR); > + /* > + * irr_pending must be true if any interrupt is pending; set it after > + * APIC_IRR to avoid race with apic_clear_irr > + */ > + apic->irr_pending = true; > } > > static inline int apic_search_irr(struct kvm_lapic *apic) > > > Thanks, > > Paolo > -- > To unsubscribe from this list: send the line "unsubscribe stable" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- 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/