KVM: X86: Fix softlockup when get the current kvmclock

[ Upstream commit e70b57a6ce4e8b92a56a615ae79bdb2bd66035e7 ]

 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_time_ref_counter+0x5a/0x80 [kvm]
  kvm_hv_process_stimers+0x120/0x5f0 [kvm]
  kvm_arch_vcpu_ioctl_run+0x4b4/0x1690 [kvm]
  kvm_vcpu_ioctl+0x33a/0x620 [kvm]
  do_vfs_ioctl+0xa1/0x5d0
  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.

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6c11a98..e023ef9 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1751,10 +1751,13 @@ static 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();