linux-kernel - Re: [PATCH v4 1/4] x86/kvm: Add "nopvspin" parameter to disable PV spinlocks

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Message-ID: <cf42de29-3460-b9ee-538f-2b1366db6ce3@oracle.com>
Date:   Tue, 8 Oct 2019 10:17:29 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     vkuznets@...hat.com, linux-hyperv@...r.kernel.org,
        kvm@...r.kernel.org, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, sashal@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, pbonzini@...hat.com,
        rkrcmar@...hat.com, sean.j.christopherson@...el.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        jgross@...e.com, sstabellini@...nel.org, peterz@...radead.org,
        Jonathan Corbet <corbet@....net>,
        "H. Peter Anvin" <hpa@...or.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 1/4] x86/kvm: Add "nopvspin" parameter to disable PV
 spinlocks


On 2019/10/7 22:46, Boris Ostrovsky wrote:
> 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.

You remember that, ok, I'll do the same for KVM.

Thanks

Zhenzhong