[PATCH v3] KVM: X86: Fix softlockup when get the current kvmclock

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

 watchdog: BUG: soft lockup - CPU#6 stuck for 22s! [qemu-system-x86:10185]
 CPU: 6 PID: 10185 Comm: qemu-system-x86 Tainted: G           OE   4.14.0-rc4+ #4
 RIP: 0010:kvm_get_time_scale+0x4e/0xa0 [kvm]
 Call Trace:
  ? get_kvmclock_ns+0xa3/0x140 [kvm]
  get_time_ref_counter+0x5a/0x80 [kvm]
  kvm_hv_process_stimers+0x120/0x5f0 [kvm]
  ? kvm_hv_process_stimers+0x120/0x5f0 [kvm]
  ? preempt_schedule+0x27/0x30
  ? ___preempt_schedule+0x16/0x18
  kvm_arch_vcpu_ioctl_run+0x4b4/0x1690 [kvm]
  ? kvm_arch_vcpu_load+0x47/0x230 [kvm]
  kvm_vcpu_ioctl+0x33a/0x620 [kvm]
  ? kvm_vcpu_ioctl+0x33a/0x620 [kvm]
  ? kvm_vm_ioctl_check_extension_generic+0x3b/0x40 [kvm]
  ? kvm_dev_ioctl+0x279/0x6c0 [kvm]
  do_vfs_ioctl+0xa1/0x5d0
  ? __fget+0x73/0xa0
  SyS_ioctl+0x79/0x90
  entry_SYSCALL_64_fastpath+0x1e/0xa9

This can be reproduced when running kvm-unit-tests/hyperv_stimer.flat and
cpu-hotplug stress simultaneously. __this_cpu_read(cpu_tsc_khz) returns 0
(set in kvmclock_cpu_down_prep()) when the pCPU is unhotplug which results
in kvm_get_time_scale() gets into an infinite loop.

This patch fixes it by treating the unhotplug pCPU as not using master clock.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 03869eb..d61dcce3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1795,10 +1795,13 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
 	get_cpu();
 
-	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
-			   &hv_clock.tsc_shift,
-			   &hv_clock.tsc_to_system_mul);
-	ret = __pvclock_read_cycles(&hv_clock, rdtsc());
+	if (__this_cpu_read(cpu_tsc_khz)) {
+		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
+				   &hv_clock.tsc_shift,
+				   &hv_clock.tsc_to_system_mul);
+		ret = __pvclock_read_cycles(&hv_clock, rdtsc());
+	} else
+		ret = ktime_get_boot_ns() + ka->kvmclock_offset;
 
 	put_cpu();
 
-- 
2.7.4