Patch "KVM: X86: Fix softlockup when get the current kvmclock" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

KVM: X86: Fix softlockup when get the current kvmclock

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-x86-fix-softlockup-when-get-the-current-kvmclock.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wanpeng Li <[email protected]>
Date: Mon, 20 Nov 2017 14:55:05 -0800
Subject: KVM: X86: Fix softlockup when get the current kvmclock

From: Wanpeng Li <[email protected]>

[ 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.

---
 arch/x86/kvm/x86.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

@@ -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();
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/kvm-x86-fix-softlockup-when-get-the-current-kvmclock.patch