Re: [PATCH v5 3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks

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On 2019/10/13 17:02, Vitaly Kuznetsov wrote:
Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> writes:
...snip
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index ef836d6..6e14bd4 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -825,18 +825,31 @@ __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))
+	/*
+	 * Disable PV qspinlocks if host kernel doesn't support
+	 * KVM_FEATURE_PV_UNHALT feature or there is only 1 vCPU.
+	 * virt_spin_lock_key is enabled to avoid lock holder
+	 * preemption issue.
+	 */
+	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) ||
+	    num_possible_cpus() == 1) {
+		pr_info("PV spinlocks disabled\n");
Why don't we need static_branch_disable(&virt_spin_lock_key) here?

Thanks for review.

I have a brief explanation in above comment area.

Boris also raised the same question in v4 and see my detailed explanation

in https://lkml.org/lkml/2019/10/6/39


Also, as you're printing the exact reason for PV spinlocks disablement
in other cases, I'd suggest separating "no host support" and "single
CPU" cases.

Will do after reaching a consensus on your first question.


  		return;
+	}
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;
  	}
- /* 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");
Nit: to make it sound better a comma is missing between 'disabled' and
'forced', or

"PV spinlocks forcefully disabled by ..." if you prefer.

Will do.

Zhenzhong





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