Re: [PATCH v7 3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks

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Hi Vitaly,

On 2019/10/22 19:36, 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 249f14a..3945aa5 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -825,18 +825,36 @@ __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))
+	/*
+	 * 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.
+	 */
+	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)) {
-		static_branch_disable(&virt_spin_lock_key);
-		return;
+		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints.\n");
+		goto out;
  	}
- /* Don't use the pvqspinlock code if there is only 1 vCPU. */
-	if (num_possible_cpus() == 1)
-		return;
+	if (num_possible_cpus() == 1) {
+		pr_info("PV spinlocks disabled, single CPU.\n");
+		goto out;
+	}
+
+	if (nopvspin) {
+		pr_info("PV spinlocks disabled, forced by \"nopvspin\" parameter.\n");
+		goto out;
+	}
+
+	pr_info("PV spinlocks enabled\n");
__pv_init_lock_hash();
  	pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
@@ -849,6 +867,8 @@ void __init kvm_spinlock_init(void)
  		pv_ops.lock.vcpu_is_preempted =
  			PV_CALLEE_SAVE(__kvm_vcpu_is_preempted);
  	}
+out:
+	static_branch_disable(&virt_spin_lock_key);
You probably need to add 'return' before 'out:' as it seems you're
disabling virt_spin_lock_key in all cases now).

virt_spin_lock_key is kept enabled in !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)
case which is the only case virt_spin_lock() optimization is used.

When PV qspinlock is enabled, virt_spin_lock() isn't called in
__pv_queued_spin_lock_slowpath() in which case we don't care
virt_spin_lock_key's value.

So adding 'return' or not are both ok, I chosed to save a line,
let me know if you prefer to add a 'return' and I'll change it.

btw: __pv_queued_spin_lock_slowpath() is alias of queued_spin_lock_slowpath()

Thanks
Zhenzhong




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