On 10/6/19 3:49 AM, Zhenzhong Duan wrote:
> On 2019/10/4 22:52, Boris Ostrovsky wrote:
>
>> On 10/3/19 10:02 AM, Zhenzhong Duan wrote:
>>> void __init kvm_spinlock_init(void)
>>> {
>>> - /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
>>> - if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
>>> - return;
>>> -
>>> - if (kvm_para_has_hint(KVM_HINTS_REALTIME))
>>> + /*
>>> + * Don't use the pvqspinlock code if no KVM_FEATURE_PV_UNHALT
>>> feature
>>> + * support, or there is REALTIME hints or only 1 vCPU.
>>> + */
>>> + if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) ||
>>> + kvm_para_has_hint(KVM_HINTS_REALTIME) ||
>>> + num_possible_cpus() == 1) {
>>> + pr_info("PV spinlocks disabled\n");
>>> return;
>>> + }
>>> - /* Don't use the pvqspinlock code if there is only 1 vCPU. */
>>> - if (num_possible_cpus() == 1)
>>> + if (nopvspin) {
>>> + pr_info("PV spinlocks disabled forced by \"nopvspin\"
>>> parameter.\n");
>>> + static_branch_disable(&virt_spin_lock_key);
>> Would it make sense to bring here the other site where the key is
>> disabled (in kvm_smp_prepare_cpus())?
>
> Thanks for point out, I'll do it. Just not clear if I should do that
> in a separate patch,
> there is a history about that code:
>
> Its original place was here and then moved to kvm_smp_prepare_cpus()
> by below commit:
> 34226b6b ("KVM: X86: Fix setup the virt_spin_lock_key before static
> key get initialized")
> which fixed jump_label_init() calling late issue.
>
> Then 8990cac6 ("x86/jump_label: Initialize static branching early")
> move jump_label_init()
> early, so commit 34226b6b could be reverted.
Which is similar to what you did earlier for Xen.
>
>>
>> (and, in fact, shouldn't all of the checks that result in early return
>> above disable the key?)
>
> I think we should enable he key for
> !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) case,
> there is lock holder preemption issue as qspinlock is fair lock,
> virt_spin_lock()
> is an optimization to that, imaging one pcpu running 10 vcpus of same
> guest
> contending a same lock.
Right. I conflated pv lock and virt_spin_lock_key, and that is wrong.
-boris
>
> For kvm_para_has_hint(KVM_HINTS_REALTIME) case, hypervisor hints there is
> no preemption and we should disable virt_spin_lock_key to use native
> qspinlock.
>
> For the UP case, we don't care virt_spin_lock_key value.
>
> For nopvspin case, we intentionally check native qspinlock code
> performance,
> compare it with PV qspinlock, etc. So virt_spin_lock() optimization
> should be disabled.
>
> Let me know if anything wrong with above understanding. Thanks
>
> Zhenzhong
>
