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    SubjectRe: [PATCH v7 3/5] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks
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    Zhenzhong Duan <zhenzhong.duan@oracle.com> writes:

    > 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.
    >

    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

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