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AJvYcCU+El4gvDk0NakkN96Vw+XNPsKqJYhXbguLPhWv//eLLYAzV2gFEOmHFNXNaX/bWd02QvA=@vger.kernel.org X-Gm-Message-State: AOJu0Yxcdo+EaNYrO0ZsOFp2VxZ7ue6s3uo8zNwzjtRukOur8LTVByF2 pAyv4pCvR24UwyE+8yW0hWvv363WsRCAS2mzcFdkn8FR7Vugh6Nc49nLKpRD3zmwVvFUGzyqTOr nztUOSw== X-Google-Smtp-Source: AGHT+IGTQp8h1kLtlKev9LhcYdbZPkpIr4HJH3gtESE8OQAlg41p7sVR2D1YicwFAm7r4ks6pGsYokdc19I= X-Received: from plbbh11.prod.google.com ([2002:a17:902:a98b:b0:2a0:dab8:9117]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:2a85:b0:2a0:8f6f:1a0d with SMTP id d9443c01a7336-2a3ee4c0fefmr34205855ad.61.1767817596631; Wed, 07 Jan 2026 12:26:36 -0800 (PST) Date: Wed, 7 Jan 2026 12:26:35 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20251230205641.4092235-1-seanjc@google.com> Message-ID: Subject: Re: [PATCH] KVM: x86: Disallow setting CPUID and/or feature MSRs if L2 is active From: Sean Christopherson To: Yosry Ahmed Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Kevin Cheng Content-Type: text/plain; charset="us-ascii" On Fri, Jan 02, 2026, Yosry Ahmed wrote: > On Tue, Dec 30, 2025 at 12:56:41PM -0800, Sean Christopherson wrote: > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h > > index fdab0ad49098..9084e0dfa15c 100644 > > --- a/arch/x86/kvm/x86.h > > +++ b/arch/x86/kvm/x86.h > > @@ -172,9 +172,9 @@ static inline void kvm_nested_vmexit_handle_ibrs(struct kvm_vcpu *vcpu) > > indirect_branch_prediction_barrier(); > > } > > > > -static inline bool kvm_vcpu_has_run(struct kvm_vcpu *vcpu) > > +static inline bool kvm_can_set_cpuid_and_feature_msrs(struct kvm_vcpu *vcpu) > > { > > - return vcpu->arch.last_vmentry_cpu != -1; > > + return vcpu->arch.last_vmentry_cpu == -1 && !is_guest_mode(vcpu); > > } > > To make this self-contained (e.g. for readers not coming from > kvm_set_cpuid()), should we add a comment here about is_guest_mode() > only possibly being true with last_vmentry_cpu == -1 if userspace does > the set CPUID, set nested state, set CPUID again dance? Ya. If this looks good, I'll add it when applying. /* * Disallow modifying CPUID and feature MSRs, which affect the core virtual CPU * model exposed to the guest and virtualized by KVM, if the vCPU has already * run or is in guest mode (L2). In both cases, KVM has already consumed the * current virtual CPU model, and doesn't support "unwinding" to react to the * new model. * * Note, the only way is_guest_mode() can be true with 'last_vmentry_cpu == -1' * is if userspace sets CPUID and feature MSRs (to enable VMX/SVM), then sets * nested state, and then attempts to set CPUID and/or feature MSRs *again*. */ static inline bool kvm_can_set_cpuid_and_feature_msrs(struct kvm_vcpu *vcpu) { return vcpu->arch.last_vmentry_cpu == -1 && !is_guest_mode(vcpu); }