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
