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Date: Thu, 8 Jun 2017 14:50:57 +0800
From: Peter Xu <peterx@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Longpeng <longpeng2@...wei.com>,
Huangweidong <weidong.huang@...wei.com>,
Gonglei <arei.gonglei@...wei.com>,
wangxin <wangxinxin.wang@...wei.com>,
Radim Krčmář <rkrcmar@...hat.com>
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@...wei.com>
> Cc: Huangweidong <weidong.huang@...wei.com>
> Cc: Gonglei <arei.gonglei@...wei.com>
> Cc: wangxin <wangxinxin.wang@...wei.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.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?)
> +
> + 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?
> }
>
> 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...
Thanks,
--
Peter Xu
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