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SubjectRe: [PATCH 2/4] KVM: VMX: avoid double list add with VT-d posted interrupts
On Thu, Jun 08, 2017 at 03:00:39AM -0400, Paolo Bonzini wrote:
>
>
> ----- Original Message -----
> > From: "Peter Xu" <peterx@redhat.com>
> > To: "Paolo Bonzini" <pbonzini@redhat.com>
> > Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, "Longpeng" <longpeng2@huawei.com>, "Huangweidong"
> > <weidong.huang@huawei.com>, "Gonglei" <arei.gonglei@huawei.com>, "wangxin" <wangxinxin.wang@huawei.com>, "Radim
> > Krčmář" <rkrcmar@redhat.com>
> > Sent: Thursday, June 8, 2017 8:50:57 AM
> > Subject: Re: [PATCH 2/4] KVM: VMX: avoid double list add with VT-d posted interrupts
> >
> > On Tue, Jun 06, 2017 at 12:57:05PM +0200, Paolo Bonzini wrote:
> > > In some cases, for example involving hot-unplug of assigned
> > > devices, pi_post_block can forget to remove the vCPU from the
> > > blocked_vcpu_list. When this happens, the next call to
> > > pi_pre_block corrupts the list.
> > >
> > > Fix this in two ways. First, check vcpu->pre_pcpu in pi_pre_block
> > > and WARN instead of adding the element twice in the list. Second,
> > > always do the list removal in pi_post_block if vcpu->pre_pcpu is
> > > set (not -1).
> > >
> > > The new code keeps interrupts disabled for the whole duration of
> > > pi_pre_block/pi_post_block. This is not strictly necessary, but
> > > easier to follow. For the same reason, PI.ON is checked only
> > > after the cmpxchg, and to handle it we just call the post-block
> > > code. This removes duplication of the list removal code.
> > >
> > > Cc: Longpeng (Mike) <longpeng2@huawei.com>
> > > Cc: Huangweidong <weidong.huang@huawei.com>
> > > Cc: Gonglei <arei.gonglei@huawei.com>
> > > Cc: wangxin <wangxinxin.wang@huawei.com>
> > > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > > ---
> > > arch/x86/kvm/vmx.c | 62
> > > ++++++++++++++++++++++--------------------------------
> > > 1 file changed, 25 insertions(+), 37 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > > index 747d16525b45..0f4714fe4908 100644
> > > --- a/arch/x86/kvm/vmx.c
> > > +++ b/arch/x86/kvm/vmx.c
> > > @@ -11236,10 +11236,11 @@ static void __pi_post_block(struct kvm_vcpu
> > > *vcpu)
> > > struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> > > struct pi_desc old, new;
> > > unsigned int dest;
> > > - unsigned long flags;
> > >
> > > do {
> > > old.control = new.control = pi_desc->control;
> > > + WARN(old.nv != POSTED_INTR_WAKEUP_VECTOR,
> > > + "Wakeup handler not enabled while the VCPU is blocked\n");
> > >
> > > dest = cpu_physical_id(vcpu->cpu);
> > >
> > > @@ -11256,14 +11257,10 @@ static void __pi_post_block(struct kvm_vcpu
> > > *vcpu)
> > > } while (cmpxchg(&pi_desc->control, old.control,
> > > new.control) != old.control);
> > >
> > > - if(vcpu->pre_pcpu != -1) {
> > > - spin_lock_irqsave(
> > > - &per_cpu(blocked_vcpu_on_cpu_lock,
> > > - vcpu->pre_pcpu), flags);
> > > + if (!WARN_ON_ONCE(vcpu->pre_pcpu == -1)) {
> > > + spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> > > list_del(&vcpu->blocked_vcpu_list);
> > > - spin_unlock_irqrestore(
> > > - &per_cpu(blocked_vcpu_on_cpu_lock,
> > > - vcpu->pre_pcpu), flags);
> > > + spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> > > vcpu->pre_pcpu = -1;
> > > }
> > > }
> > > @@ -11283,7 +11280,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
> > > */
> > > static int pi_pre_block(struct kvm_vcpu *vcpu)
> > > {
> > > - unsigned long flags;
> > > unsigned int dest;
> > > struct pi_desc old, new;
> > > struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> > > @@ -11293,34 +11289,20 @@ static int pi_pre_block(struct kvm_vcpu *vcpu)
> > > !kvm_vcpu_apicv_active(vcpu))
> > > return 0;
> > >
> > > - vcpu->pre_pcpu = vcpu->cpu;
> > > - spin_lock_irqsave(&per_cpu(blocked_vcpu_on_cpu_lock,
> > > - vcpu->pre_pcpu), flags);
> > > - list_add_tail(&vcpu->blocked_vcpu_list,
> > > - &per_cpu(blocked_vcpu_on_cpu,
> > > - vcpu->pre_pcpu));
> > > - spin_unlock_irqrestore(&per_cpu(blocked_vcpu_on_cpu_lock,
> > > - vcpu->pre_pcpu), flags);
> > > + WARN_ON(irqs_disabled());
> > > + local_irq_disable();
> > > + if (!WARN_ON_ONCE(vcpu->pre_pcpu != -1)) {
> > > + vcpu->pre_pcpu = vcpu->cpu;
> > > + spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> > > + list_add_tail(&vcpu->blocked_vcpu_list,
> > > + &per_cpu(blocked_vcpu_on_cpu,
> > > + vcpu->pre_pcpu));
> > > + spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> > > + }
> > >
> > > do {
> > > old.control = new.control = pi_desc->control;
> > >
> > > - /*
> > > - * We should not block the vCPU if
> > > - * an interrupt is posted for it.
> > > - */
> > > - if (pi_test_on(pi_desc) == 1) {
> > > - spin_lock_irqsave(&per_cpu(blocked_vcpu_on_cpu_lock,
> > > - vcpu->pre_pcpu), flags);
> > > - list_del(&vcpu->blocked_vcpu_list);
> > > - spin_unlock_irqrestore(
> > > - &per_cpu(blocked_vcpu_on_cpu_lock,
> > > - vcpu->pre_pcpu), flags);
> > > - vcpu->pre_pcpu = -1;
> > > -
> > > - return 1;
> >
> > [1]
> >
> > > - }
> > > -
> > > WARN((pi_desc->sn == 1),
> > > "Warning: SN field of posted-interrupts "
> > > "is set before blocking\n");
> > > @@ -11345,7 +11327,12 @@ static int pi_pre_block(struct kvm_vcpu *vcpu)
> > > } while (cmpxchg(&pi_desc->control, old.control,
> > > new.control) != old.control);
> > >
> > > - return 0;
> > > + /* We should not block the vCPU if an interrupt is posted for it. */
> > > + if (pi_test_on(pi_desc) == 1)
> > > + __pi_post_block(vcpu);
> >
> > A question on when pi_test_on() is set:
> >
> > The old code will return 1 if detected (ses [1]), while the new code
> > does not. Would that matter? (IIUC that decides whether the vcpu will
> > continue to run?)
>
> The new code does, because __pi_post_block resets vcpu->pre_pcpu to -1.

Sorry I overlook that. :-)

>
> > > +
> > > + local_irq_enable();
> > > + return (vcpu->pre_pcpu == -1);
> >
> > Above we have:
> >
> > if (!WARN_ON_ONCE(vcpu->pre_pcpu != -1)) {
> > vcpu->pre_pcpu = vcpu->cpu;
> > ...
> > }
> >
> > Then can here vcpu->pre_pcpu really be -1?
>
> See above. :)

Yes. Then there's no problem.

>
> > > }
> > >
> > > static int vmx_pre_block(struct kvm_vcpu *vcpu)
> > > @@ -11361,12 +11348,13 @@ static int vmx_pre_block(struct kvm_vcpu *vcpu)
> > >
> > > static void pi_post_block(struct kvm_vcpu *vcpu)
> > > {
> > > - if (!kvm_arch_has_assigned_device(vcpu->kvm) ||
> > > - !irq_remapping_cap(IRQ_POSTING_CAP) ||
> > > - !kvm_vcpu_apicv_active(vcpu))
> > > + if (vcpu->pre_pcpu == -1)
> > > return;
> > >
> > > + WARN_ON(irqs_disabled());
> > > + local_irq_disable();
> > > __pi_post_block(vcpu);
> > > + local_irq_enable();
> > > }
> > >
> > > static void vmx_post_block(struct kvm_vcpu *vcpu)
> > > --
> > > 2.13.0
> > >
> > >
> >
> > A general question to pre_block/post_block handling for PI:
> >
> > I see that we are handling PI logic mostly in four places:
> >
> > vmx_vcpu_pi_{load|put}
> > pi_{pre_post}_block
> >
> > But do we really need the pre_block/post_block handling? Here's how I
> > understand when vcpu blocked:
> >
> > - vcpu_block
> > - ->pre_block
> > - kvm_vcpu_block [1]
> > - schedule()
> > - kvm_sched_out
> > - vmx_vcpu_pi_put [3]
> > - (another process working) ...
> > - kvm_sched_in
> > - vmx_vcpu_pi_load [4]
> > - ->post_block [2]
> >
> > If so, [1] & [2] will definitely be paired with [3] & [4], then why we
> > need [3] & [4] at all?
> >
> > (Though [3] & [4] will also be used when preemption happens, so they
> > are required)
> >
> > Please kindly figure out if I missed anything important...
>
> Oh, I see what you mean: set up the wakeup handler in vmx_vcpu_pi_put
> and rely on PI.ON to wake up the sleeping process immediately. That
> should be feasible, but overall I like the current pre_block/post_block
> structure, and I think it's simpler. The only thing to be careful about
> is leaving the IRTE unmodified when scheduling out a blocked VCPU, which
> is cleaned up and simplified in patch 3.
>
> So I understand that the state may seem a bit too complicated as
> of this patch, but hopefully the next two make it clearer.

After re-read the codes and patches I got the point. Indeed current
way should be clearer since pre/post_block are mostly handling NV/DST
while pi_load/put are for SN bit. Thanks!

--
Peter Xu

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