Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

[Sean Christopherson <seanjc@google.com>]

On Sat, Sep 30, 2023, David Woodhouse wrote:
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 40edf4d1974c..75586da134b3 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -134,9 +134,23 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
>  {
>  	struct kvm_vcpu *vcpu = container_of(timer, struct kvm_vcpu,
>  					     arch.xen.timer);
> +	struct kvm_xen_evtchn e;
> +	int rc;
> +
>  	if (atomic_read(&vcpu->arch.xen.timer_pending))
>  		return HRTIMER_NORESTART;
>  
> +	e.vcpu_id = vcpu->vcpu_id;
> +	e.vcpu_idx = vcpu->vcpu_idx;
> +	e.port = vcpu->arch.xen.timer_virq;
> +	e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
> +
> +	rc = kvm_xen_set_evtchn_fast(&e, vcpu->kvm);
> +	if (rc != -EWOULDBLOCK) {
> +		vcpu->arch.xen.timer_expires = 0;
> +		return HRTIMER_NORESTART;
> +	}
> +
>  	atomic_inc(&vcpu->arch.xen.timer_pending);
>  	kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
>  	kvm_vcpu_kick(vcpu);
> @@ -146,6 +160,14 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
>  
>  static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, s64 delta_ns)
>  {
> +	/*
> +	 * Avoid races with the old timer firing. Checking timer_expires
> +	 * to avoid calling hrtimer_cancel() will only have false positives
> +	 * so is fine.
> +	 */
> +	if (vcpu->arch.xen.timer_expires)
> +		hrtimer_cancel(&vcpu->arch.xen.timer);
> +
>  	atomic_set(&vcpu->arch.xen.timer_pending, 0);
>  	vcpu->arch.xen.timer_expires = guest_abs;
>  
> @@ -1019,9 +1041,36 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  		break;
>  
>  	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> +		/*
> +		 * Ensure a consistent snapshot of state is captured, with a
> +		 * timer either being pending, or the event channel delivered
> +		 * to the corresponding bit in the shared_info. Not still
> +		 * lurking in the timer_pending flag for deferred delivery.
> +		 * Purely as an optimisation, if the timer_expires field is
> +		 * zero, that means the timer isn't active (or even in the
> +		 * timer_pending flag) and there is no need to cancel it.
> +		 */
> +		if (vcpu->arch.xen.timer_expires) {
> +			hrtimer_cancel(&vcpu->arch.xen.timer);
> +			kvm_xen_inject_timer_irqs(vcpu);

This has an obvious-in-hindsight recursive deadlock bug.  If KVM actually needs
to inject a timer IRQ, and the fast path fails, i.e. the gpc is invalid,
kvm_xen_set_evtchn() will attempt to acquire xen.xen_lock, which is already held.

Not sure if I sucked at testing before, or if I just got "lucky" on a random run.

 ============================================
 WARNING: possible recursive locking detected
 6.8.0-smp--5e10b4d51d77-drs #232 Tainted: G           O      
 --------------------------------------------
 xen_shinfo_test/250013 is trying to acquire lock:
 ffff938c9930cc30 (&kvm->arch.xen.xen_lock){+.+.}-{3:3}, at: kvm_xen_set_evtchn+0x74/0x170 [kvm]
 
 but task is already holding lock:
 ffff938c9930cc30 (&kvm->arch.xen.xen_lock){+.+.}-{3:3}, at: kvm_xen_vcpu_get_attr+0x38/0x250 [kvm]
 
 other info that might help us debug this:
  Possible unsafe locking scenario:
        CPU0
        ----
   lock(&kvm->arch.xen.xen_lock);
   lock(&kvm->arch.xen.xen_lock);
 
  *** DEADLOCK ***
  May be due to missing lock nesting notation
 2 locks held by xen_shinfo_test/250013:
  #0: ffff9228863f21a8 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x8f/0x5b0 [kvm]
  #1: ffff938c9930cc30 (&kvm->arch.xen.xen_lock){+.+.}-{3:3}, at: kvm_xen_vcpu_get_attr+0x38/0x250 [kvm]
 
 stack backtrace:
 CPU: 128 PID: 250013 Comm: xen_shinfo_test Tainted: G           O       6.8.0-smp--5e10b4d51d77-drs #232
 Hardware name: Google, Inc.                                                       Arcadia_IT_80/Arcadia_IT_80, BIOS 34.30.0 07/27/2023
 Call Trace:
  <TASK>
  dump_stack_lvl+0x69/0xa0
  dump_stack+0x14/0x20
  print_deadlock_bug+0x2af/0x2c0
  __lock_acquire+0x13f7/0x2e30
  lock_acquire+0xd4/0x220
  __mutex_lock+0x6a/0xa60
  mutex_lock_nested+0x1f/0x30
  kvm_xen_set_evtchn+0x74/0x170 [kvm]
  kvm_xen_vcpu_get_attr+0x136/0x250 [kvm]
  kvm_arch_vcpu_ioctl+0x942/0x1130 [kvm]
  kvm_vcpu_ioctl+0x484/0x5b0 [kvm]
  __se_sys_ioctl+0x7a/0xc0
  __x64_sys_ioctl+0x21/0x30
  do_syscall_64+0x82/0x160
  entry_SYSCALL_64_after_hwframe+0x63/0x6b
 RIP: 0033:0x460eab