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

[Zhenzhong Duan <zhenzhong.duan@oracle.com>]

On 2019/10/23 5:03, Sean Christopherson wrote:
> 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.

Sure, will do in next version.

Thanks

Zhenzhong