Re: [PATCH v6 3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




On 2019/10/21 19:14, Vitaly Kuznetsov wrote:
index 249f14a..e9c76d8 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -825,18 +825,44 @@ __visible bool __kvm_vcpu_is_preempted(long cpu)
   */
  void __init kvm_spinlock_init(void)
  {
-	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
-	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
+	/*
+	 * PV spinlocks is disabled if no host side support, then native
+	 * qspinlock will be used. As native qspinlock is a fair lock, there is
+	 * lock holder preemption issue using it in a guest, imaging one pCPU
+	 * running 10 vCPUs of same guest contending same lock.
+	 *
+	 * virt_spin_lock() is introduced as an optimization for that scenario
+	 * which is enabled by virt_spin_lock_key key. To use that optimization,
+	 * virt_spin_lock_key isn't disabled here.
+	 */
My take (if I properly understood what you say) would be:

"In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
preferred over native qspinlock when vCPU is preempted."

Yes, that's what I mean, maybe I didn't explain clearly due to my pool english,

I'll use your explanation instead.


+	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
+		pr_info("PV spinlocks disabled, no host support.\n");
  		return;
+	}
+ /*
+	 * Disable PV qspinlock and use native qspinlock when dedicated pCPUs
+	 * are available.
+	 */
  	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
+		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
+		static_branch_disable(&virt_spin_lock_key);
+		return;
+	}
+
+	if (num_possible_cpus() == 1) {
+		pr_info("PV spinlocks disabled, single CPU.\n");
  		static_branch_disable(&virt_spin_lock_key);
  		return;
  	}
- /* Don't use the pvqspinlock code if there is only 1 vCPU. */
-	if (num_possible_cpus() == 1)
+	if (nopvspin) {
+		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
+		static_branch_disable(&virt_spin_lock_key);
  		return;
You could've replaced this 'static_branch_disable(); return;' pattern
with a goto to the end of the function to save a few lines but this
looks good anyways.

Reviewed-by: Vitaly Kuznetsov<vkuznets@xxxxxxxxxx>

Ok, will do, thanks for review.

Zhenzhong




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux