Messages in this thread | | | Subject | Re: [PATCH v7 3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks | From | Zhenzhong Duan <> | Date | Wed, 23 Oct 2019 09:36:00 +0800 |
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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
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