[PATCH v2] KVM: X86: Fix softlockup when get the current kvmclock timestamp

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

 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 skipping to fill the hv_clock when the pCPU is offline.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
 * avoid infinite loop

 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 03869eb..d2507c6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1259,6 +1259,9 @@ static void kvm_get_time_scale(uint64_t scaled_hz, uint64_t base_hz,
 	uint64_t tps64;
 	uint32_t tps32;
 
+	if (unlikely(base_hz == 0))
+		return;
+
 	tps64 = base_hz;
 	scaled64 = scaled_hz;
 	while (tps64 > scaled64*2 || tps64 & 0xffffffff00000000ULL) {
-- 
2.7.4