linux-kernel - Re: [PATCH 01/12] KVM: x86: Add a framework for enabling KVM-governed x86 features

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Date:   Fri, 30 Jun 2023 16:01:38 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/12] KVM: x86: Add a framework for enabling
 KVM-governed x86 features

On Fri, Feb 17, 2023 at 03:10:11PM -0800, Sean Christopherson wrote:
>+static __always_inline void kvm_governed_feature_set(struct kvm_vcpu *vcpu,
>+						     unsigned int x86_feature)
>+{
>+	BUILD_BUG_ON(KVM_NR_GOVERNED_FEATURES >
>+		     sizeof(vcpu->arch.governed_features.enabled) * BITS_PER_BYTE);
>+
>+	vcpu->arch.governed_features.enabled |= kvm_governed_feature_bit(x86_feature);
>+}
>+
>+static __always_inline void kvm_governed_feature_check_and_set(struct kvm_vcpu *vcpu,
>+							       unsigned int x86_feature)
>+{
>+	if (guest_cpuid_has(vcpu, x86_feature))

Most callers in this series are conditional on either boot_cpu_has() or some
local variables. Can we convert them to kvm_cpu_cap_has() and incorporate them
within this function? i.e.,

	if (kvm_cpu_cap_has(x86_feature) && guest_cpuid_has(vcpu, x86_feature))


The benefits of doing so are
1. callers needn't repeat

	if (kvm_cpu_cap_has(x86_feature))
		kvm_governed_feature_check_and_set(x86_feature)

2. this fits the idea better that guests can use a governed feature only if host
   supports it _and_ QEMU exposes it to the guest.

>+		kvm_governed_feature_set(vcpu, x86_feature);
>+}
>+