linux-kernel - Re: [PATCH v4 04/17] kvm: x86: Add support for activate/de-activate APICv at runtime

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Date:   Mon, 4 Nov 2019 19:22:43 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "rkagan@...tuozzo.com" <rkagan@...tuozzo.com>,
        "graf@...zon.com" <graf@...zon.com>,
        "jschoenh@...zon.de" <jschoenh@...zon.de>,
        "karahmed@...zon.de" <karahmed@...zon.de>,
        "rimasluk@...zon.com" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v4 04/17] kvm: x86: Add support for activate/de-activate
 APICv at runtime

Paolo,

On 11/2/19 4:52 AM, Paolo Bonzini wrote:
> On 01/11/19 23:41, Suthikulpanit, Suravee wrote:
>> +void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
>> +{
>> +	if (activate) {
>> +		if (!test_and_clear_bit(bit, &kvm->arch.apicv_deact_msk) ||
>> +		    !kvm_apicv_activated(kvm))
>> +			return;
>> +	} else {
>> +		if (test_and_set_bit(bit, &kvm->arch.apicv_deact_msk) ||
>> +		    kvm_apicv_activated(kvm))
>> +			return;
>> +	}
>> +
>> +	kvm_make_all_cpus_request(kvm, KVM_REQ_APICV_UPDATE);
>> +}
>> +EXPORT_SYMBOL_GPL(kvm_request_apicv_update);
>> +
> 
> It's worth documenting the locking requirements of
> kvm_request_apicv_update (it can also be negative requirements, such as
> "don't hold any lock"), because kvm_make_all_cpus_request is a somewhat
> deadlock-prone API.

Currently, I have a comment in the svm_request_update_avic(), where it 
calls kvm_request_apicv_update. I'll move it here instead and enhance 
the comment.

Thanks,
Suravee