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

[Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>]

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