linux-kernel - Re: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

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Message-ID: <CANRm+Cy1FBS6sP6q4uumsvgMp11zbirCbjZvaPRBgzK-5K-Xsg@mail.gmail.com>
Date:	Wed, 29 Jun 2016 06:55:00 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	yunhong jiang <yunhong.jiang@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...el.com>
Subject: Re: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

2016-06-29 4:07 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 28/06/2016 20:34, yunhong jiang wrote:
>> Paolo, thanks for reply.
>>
>> Which race window you are talking about? Is it the
>> kvm_lapic_switch_to_hv_timer()? If yes, hope we will not forgot it once the
>> lapic timer is not pinned anymore in future.
>
> Yes, it's that one.  This is a good point against using
> local_irq_save/restore.
>
> Paolo

How about something like below(untested, just want to know if this is
what you mean):

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index fdc05ae..adf8e32 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1361,6 +1361,33 @@ void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_lapic_expired_hv_timer);

+static void start_hv_tscdeadline(struct kvm_lapic *apic)
+{
+ unsigned long flags;
+ u64 tscdeadline = apic->lapic_timer.tscdeadline;
+
+ local_irq_save(flags);
+ if (kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline)) {
+ if (apic->lapic_timer.hv_timer_in_use) {
+ apic->lapic_timer.hv_timer_in_use = false;
+ kvm_x86_ops->cancel_hv_timer(apic->vcpu);
+ }
+ start_sw_tscdeadline(apic);
+ } else {
+ apic->lapic_timer.hv_timer_in_use = true;
+ hrtimer_cancel(&apic->lapic_timer.timer);
+
+ /* In case the sw timer triggered in the window */
+ if (atomic_read(&apic->lapic_timer.pending)) {
+ apic->lapic_timer.hv_timer_in_use = false;
+ kvm_x86_ops->cancel_hv_timer(apic->vcpu);
+ }
+ }
+ local_irq_restore(flags);
+ trace_kvm_hv_timer_state(apic->vcpu->vcpu_id,
+ apic->lapic_timer.hv_timer_in_use);
+}
+
 void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu)
 {
  struct kvm_lapic *apic = vcpu->arch.apic;
@@ -1368,22 +1395,8 @@ void kvm_lapic_switch_to_hv_timer(struct kvm_vcpu *vcpu)
  WARN_ON(apic->lapic_timer.hv_timer_in_use);

  if (apic_lvtt_tscdeadline(apic) &&
-    !atomic_read(&apic->lapic_timer.pending)) {
- u64 tscdeadline = apic->lapic_timer.tscdeadline;
-
- if (!kvm_x86_ops->set_hv_timer(vcpu, tscdeadline)) {
- apic->lapic_timer.hv_timer_in_use = true;
- hrtimer_cancel(&apic->lapic_timer.timer);
-
- /* In case the sw timer triggered in the window */
- if (atomic_read(&apic->lapic_timer.pending)) {
- apic->lapic_timer.hv_timer_in_use = false;
- kvm_x86_ops->cancel_hv_timer(apic->vcpu);
- }
- }
- trace_kvm_hv_timer_state(vcpu->vcpu_id,
- apic->lapic_timer.hv_timer_in_use);
- }
+    !atomic_read(&apic->lapic_timer.pending))
+ start_hv_tscdeadline(apic);
 }
 EXPORT_SYMBOL_GPL(kvm_lapic_switch_to_hv_timer);

@@ -1451,15 +1464,9 @@ static void start_apic_timer(struct kvm_lapic *apic)
    ktime_to_ns(ktime_add_ns(now,
  apic->lapic_timer.period)));
  } else if (apic_lvtt_tscdeadline(apic)) {
- /* lapic timer in tsc deadline mode */
- u64 tscdeadline = apic->lapic_timer.tscdeadline;
-
- if (kvm_x86_ops->set_hv_timer &&
-    !kvm_x86_ops->set_hv_timer(apic->vcpu, tscdeadline)) {
- apic->lapic_timer.hv_timer_in_use = true;
- trace_kvm_hv_timer_state(apic->vcpu->vcpu_id,
- apic->lapic_timer.hv_timer_in_use);
- } else
+ if (kvm_x86_ops->set_hv_timer)
+ start_hv_tscdeadline(apic);
+ else
  start_sw_tscdeadline(apic);
  }
 }