[PATCH v3 2/2] fix hardlockup when waking VM after long suspend

[fuqiang wang <fuqiang.wng@gmail.com>]

When a virtual machine uses the HV timer during suspend, the KVM timer does
not advance. Upon waking after a long period, there may be a significant
gap between target_expiration and the current time. Since each timer
expiration only advances target_expiration by one period, the expiration
handler can be invoked repeatedly to catch up.

Prior to the previous patch, if the advanced target_expiration remained
less than the current time, tscdeadline could be set to a negative value.
This would cause HV timer setup to fail and fallback to the SW timer. After
switching to SW timer, apic_timer_fn could be repeatedly executed within a
single clock interrupt handler, resulting in a hardlockup:

  NMI watchdog: Watchdog detected hard LOCKUP on cpu 45
  ...
  RIP: 0010:advance_periodic_target_expiration+0x4d/0x80 [kvm]
  ...
  RSP: 0018:ff4f88f5d98d8ef0 EFLAGS: 00000046
  RAX: fff0103f91be678e RBX: fff0103f91be678e RCX: 00843a7d9e127bcc
  RDX: 0000000000000002 RSI: 0052ca4003697505 RDI: ff440d5bfbdbd500
  RBP: ff440d5956f99200 R08: ff2ff2a42deb6a84 R09: 000000000002a6c0
  R10: 0122d794016332b3 R11: 0000000000000000 R12: ff440db1af39cfc0
  R13: ff440db1af39cfc0 R14: ffffffffc0d4a560 R15: ff440db1af39d0f8
  FS:  00007f04a6ffd700(0000) GS:ff440db1af380000(0000) knlGS:000000e38a3b8000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 000000d5651feff8 CR3: 000000684e038002 CR4: 0000000000773ee0
  PKRU: 55555554
  Call Trace:
   <IRQ>
   apic_timer_fn+0x31/0x50 [kvm]
   __hrtimer_run_queues+0x100/0x280
   hrtimer_interrupt+0x100/0x210
   ? ttwu_do_wakeup+0x19/0x160
   smp_apic_timer_interrupt+0x6a/0x130
   apic_timer_interrupt+0xf/0x20
   </IRQ>

With the previous patch applied, HV timer no longer falls back to SW timer.
Additionally, while target_expiration is catching up to the current time,
the VMX-preemption timer is set to 0 before each VM entry. According to
Intel SDM 27.7.4 ("VMX-Preemption Timer"), if the timer has already expired
at VM entry, a VM exit occurs before any guest instruction executes. As a
result, the guest cannot run instructions during this period and cannot
reach vcpu_block() to switch to the SW timer, preventing hardlockup.

However, unnecessary repeated catch-ups should still be avoided. Therefore,
if the advanced target_expiration is still less than the current time, we
immediately catch up to the current time in the handler.

Signed-off-by: fuqiang wang <fuqiang.wng@gmail.com>
---
 arch/x86/kvm/lapic.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index fa07a303767c..307e2d6c3450 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2140,17 +2140,25 @@ static void advance_periodic_target_expiration(struct kvm_lapic *apic)
 	apic->lapic_timer.target_expiration =
 		ktime_add_ns(apic->lapic_timer.target_expiration,
 				apic->lapic_timer.period);
-	delta = ktime_sub(apic->lapic_timer.target_expiration, now);
 
 	/*
-	 * Don't adjust the tscdeadline if the next period has already expired,
-	 * e.g. due to software overhead resulting in delays larger than the
-	 * period.  Blindly adding a negative delta could cause the deadline to
-	 * become excessively large due to the deadline being an unsigned value.
+	 * When the vm is suspend, the hv timer also stops advancing. After it
+	 * is resumed, this may result in a large delta. If the
+	 * target_expiration only advances by one period each time, it will
+	 * cause KVM to frequently handle timer expirations.
 	 */
+	if (apic->lapic_timer.period > 0 &&
+	    ktime_before(apic->lapic_timer.target_expiration, now))
+		apic->lapic_timer.target_expiration = now;
+
+	delta = ktime_sub(apic->lapic_timer.target_expiration, now);
 	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl);
-	if (delta > 0)
-		apic->lapic_timer.tscdeadline += nsec_to_cycles(apic->vcpu, delta);
+	/*
+	 * Note: delta must not be negative. Otherwise, blindly adding a
+	 * negative delta could cause the deadline to become excessively large
+	 * due to the deadline being an unsigned value.
+	 */
+	apic->lapic_timer.tscdeadline += nsec_to_cycles(apic->vcpu, delta);
 }
 
 static void start_sw_period(struct kvm_lapic *apic)
@@ -2980,7 +2988,7 @@ static enum hrtimer_restart apic_timer_fn(struct hrtimer *data)
 
 	if (lapic_is_periodic(apic)) {
 		advance_periodic_target_expiration(apic);
-		hrtimer_add_expires_ns(&ktimer->timer, ktimer->period);
+		hrtimer_set_expires(&ktimer->timer, ktimer->target_expiration);
 		return HRTIMER_RESTART;
 	} else
 		return HRTIMER_NORESTART;
-- 
2.47.0