From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH RFC] KVM: Fix race in apic->pending_events processing Date: Thu, 30 May 2013 08:31:11 +0200 Message-ID: <51A6F22F.2000600@redhat.com> References: <20130526130031.GS4725@redhat.com> <51A48D53.7070204@redhat.com> <20130528125613.GB3326@redhat.com> <51A4B5CA.9070109@redhat.com> <20130528150057.GA6891@redhat.com> <51A4DC63.2040906@redhat.com> <20130530012048.GB20766@redhat.com> <51A6E671.8070408@redhat.com> <20130530060150.GA28173@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, Jan Kiszka To: Gleb Natapov Return-path: Received: from mail-ea0-f182.google.com ([209.85.215.182]:34694 "EHLO mail-ea0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S967568Ab3E3GbY (ORCPT ); Thu, 30 May 2013 02:31:24 -0400 Received: by mail-ea0-f182.google.com with SMTP id r16so6269430ead.13 for ; Wed, 29 May 2013 23:31:23 -0700 (PDT) In-Reply-To: <20130530060150.GA28173@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Il 30/05/2013 08:01, Gleb Natapov ha scritto: > On Thu, May 30, 2013 at 07:41:05AM +0200, Paolo Bonzini wrote: >> Il 30/05/2013 03:20, Gleb Natapov ha scritto: >>> On Tue, May 28, 2013 at 06:33:39PM +0200, Paolo Bonzini wrote: >>>> Il 28/05/2013 17:00, Gleb Natapov ha scritto: >>>>> On Tue, May 28, 2013 at 03:48:58PM +0200, Paolo Bonzini wrote: >>>>>> Il 28/05/2013 14:56, Gleb Natapov ha scritto: >>>>>>>>> else >>>>>>>>> vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED; >>>>>>>>> } >>>>>>>>> - if (test_and_clear_bit(KVM_APIC_SIPI, &apic->pending_events) && >>>>>>>>> + /* >>>>>>>>> + * Note that we may get another INIT+SIPI sequence right here; process >>>>>>>>> + * the INIT first. Assumes that there are only KVM_APIC_INIT/SIPI. >>>>>>>>> + */ >>>>>>>>> + if (cmpxchg(&apic->pending_events, KVM_APIC_SIPI, 0) == KVM_APIC_SIPI && >>>>>>>>> vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) { >>>>>>> Because pending_events can be INIT/SIPI at this point and it should be >>>>>>> interpreted as: do SIPI and ignore INIT (atomically). >>>>>> >>>>>> My patch does "do another INIT (which will have no effect) and do SIPI >>>>>> after that INIT", which is different but has almost the same effect. >>>>>> If pending_events is INIT/SIPI, it ignores the SIPI for now and lets >>>>>> the next iteration of kvm_apic_accept_events do both. The difference >>>>>> would be that in a carefully-timed sequence of interrupts >>>>>> >>>>> You assume that the next processing will actually happen, but this is >>>>> not necessary the case. >>>> >>>> Why not? The INIT and SIPI that have just been sent have kicked the >>>> VCPU again. >>> >>> kick is a nop if vcpu thread is not in a halt or in a guest. >> >> But the KVM_REQ_EVENT request will be caught at: >> >> if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests >> || need_resched() || signal_pending(current)) { >> vcpu->mode = OUTSIDE_GUEST_MODE; >> smp_wmb(); >> local_irq_enable(); >> preempt_enable(); >> r = 1; >> goto cancel_injection; >> } >> >> and the entry will be canceled. I was wrong: we exit immediately because state is KVM_MP_STATE_INIT_RECEIVED. But then... > But vcpu may be in non running state so we will not get here. ... vcpu_enter_guest will return 1 and __vcpu_run goes around the while loop once more (modulo pending signals of course). On the next iteration __vcpu_run will call kvm_vcpu_block, which calls kvm_arch_vcpu_runnable. kvm_arch_vcpu_runnable returns true because kvm_apic_has_events(vcpu) is also true. This will set KVM_REQ_UNHALT, call kvm_apic_accept_events again and do the INIT+SIPI. Paolo