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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Oct 22, 2019 at 08:46:46PM +0800, Zhenzhong Duan wrote:
> Hi Vitaly,
> 
> On 2019/10/22 19:36, Vitaly Kuznetsov wrote:
> 
> >Zhenzhong Duan<zhenzhong.duan@oracle.com>  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.

It'd be worth adding a comment here if you end up spinning another version
to change the logging prefix.  The logic is sound and I like the end
result, but I had the same knee jerk "this can't be right!?!?" reaction as
Vitaly.