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

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Date:   Sat, 2 Nov 2019 10:52:18 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....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

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.

Again, something I'll check after a more thorough review.

Paolo