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AJvYcCUodYmvMXzdVAi8Xdq32xumr99VbPh+4v9772d1jiA02dSxLQK4/o0POmdukzpbBn4Zxa8=@vger.kernel.org X-Gm-Message-State: AOJu0YyTAWbTN3ujStH9pR49z6ypY+HynaB5SXjeGeiv7byMnFOSASjh CPrdcxelFcfAYJtOF65dEoAI/sVkhnZ9liTj/pNKNlN6c8ECe2ERNWT6SAQ3VqHIx7U2nfgkNgO SGw== X-Google-Smtp-Source: AGHT+IEfEGYrf9Kp0+2ubym5ip7jGdmma8H6AvjPeN7XwsvYzFyoI8GJuVmio+ck2iYi43MOyqOFlyy+ZwQ= X-Received: from pjbdy11.prod.google.com ([2002:a17:90b:6cb:b0:2eb:12c1:bf8f]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:2e48:b0:2ee:6736:8512 with SMTP id 98e67ed59e1d1-2f28fb6f983mr27557050a91.12.1734393902184; Mon, 16 Dec 2024 16:05:02 -0800 (PST) Date: Mon, 16 Dec 2024 16:05:00 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20241128013424.4096668-1-seanjc@google.com> <20241128013424.4096668-51-seanjc@google.com> Message-ID: Subject: Re: [PATCH v3 50/57] KVM: x86: Replace (almost) all guest CPUID feature queries with cpu_caps From: Sean Christopherson To: Chao Gao Cc: Paolo Bonzini , Vitaly Kuznetsov , Jarkko Sakkinen , kvm@vger.kernel.org, linux-sgx@vger.kernel.org, linux-kernel@vger.kernel.org, Maxim Levitsky , Hou Wenlong , Xiaoyao Li , Kechen Lu , Oliver Upton , Binbin Wu , Yang Weijiang , Robert Hoo Content-Type: text/plain; charset="us-ascii" On Fri, Dec 13, 2024, Chao Gao wrote: > On Wed, Nov 27, 2024 at 05:34:17PM -0800, Sean Christopherson wrote: > >diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > >index d3c3e1327ca1..8d088a888a0d 100644 > >--- a/arch/x86/kvm/cpuid.c > >+++ b/arch/x86/kvm/cpuid.c > >@@ -416,7 +416,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) > > * and can install smaller shadow pages if the host lacks 1GiB support. > > */ > > allow_gbpages = tdp_enabled ? boot_cpu_has(X86_FEATURE_GBPAGES) : > >- guest_cpuid_has(vcpu, X86_FEATURE_GBPAGES); > >+ guest_cpu_cap_has(vcpu, X86_FEATURE_GBPAGES); > > guest_cpu_cap_change(vcpu, X86_FEATURE_GBPAGES, allow_gbpages); > > > > best = kvm_find_cpuid_entry(vcpu, 1); > >@@ -441,7 +441,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) > > > > #define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f) > > vcpu->arch.cr4_guest_rsvd_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_) | > >- __cr4_reserved_bits(guest_cpuid_has, vcpu); > >+ __cr4_reserved_bits(guest_cpu_cap_has, vcpu); > > So, actually, __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_) can be dropped. > Is there any reason to keep it? It makes perfect sense to just look up the > guest cpu_caps given it already takes KVM caps into consideration. Hmm, good point. I agree that that keeping the __kvm_cpu_cap_has() checks is unnecessary, though I'm tempted to turn it into a WARN. E.g. to guard against stuffing a feature into cpu_caps without thinking through the implications. diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index edef30359c19..3cbf384aeb7a 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -460,9 +460,16 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) kvm_pmu_refresh(vcpu); + vcpu->arch.cr4_guest_rsvd_bits = __cr4_reserved_bits(guest_cpu_cap_has, vcpu); + /* + * KVM's capabilities are incorporated into the vCPU's capabilities, + * and letting the guest to use a CR4-based feature that KVM doesn't + * support isn't allowed as KVM either needs to explicitly emulate the + * feature or set the CR4 bit in hardware. + */ #define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f) - vcpu->arch.cr4_guest_rsvd_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_) | - __cr4_reserved_bits(guest_cpu_cap_has, vcpu); + WARN_ON_ONCE(~vcpu->arch.cr4_guest_rsvd_bits & + __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_)); #undef __kvm_cpu_cap_has kvm_hv_set_cpuid(vcpu, kvm_cpuid_has_hyperv(vcpu));