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

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Date:   Tue, 22 Oct 2019 20:46:46 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        pbonzini@...hat.com, rkrcmar@...hat.com,
        sean.j.christopherson@...el.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, boris.ostrovsky@...cle.com,
        jgross@...e.com, peterz@...radead.org, will@...nel.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org,
        mikelley@...rosoft.com, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, sashal@...nel.org,
        Jonathan Corbet <corbet@....net>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v7 3/5] x86/kvm: Add "nopvspin" parameter to disable PV
 spinlocks

Hi Vitaly,

On 2019/10/22 19:36, Vitaly Kuznetsov wrote:

> Zhenzhong Duan<zhenzhong.duan@...cle.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.

btw: __pv_queued_spin_lock_slowpath() is alias of queued_spin_lock_slowpath()

Thanks
Zhenzhong