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    SubjectRe: [PATCH v4 1/4] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks
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    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())?

    (and, in fact, shouldn't all of the checks that result in early return
    above disable the key?)

    -boris

    > return;
    > + }
    > + pr_info("PV spinlocks enabled\n");
    >
    > __pv_init_lock_hash();
    > pv_ops.lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
    >

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