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

[Vitaly Kuznetsov]

Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> writes:

> 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.
>

True, my bad: I though we still need it enabled for something.

> 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.

No, please ignore.

>
> btw: __pv_queued_spin_lock_slowpath() is alias of queued_spin_lock_slowpath()
>
> Thanks
> Zhenzhong
>

-- 
Vitaly