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From:   Jim Mattson <jmattson@google.com>
Date:   Tue, 15 Jun 2021 15:30:33 -0700
Message-ID: <CALMp9eRxA0zk9abXp-YwGJwO2QX-EvPfkS=CCCeLKFAcPx7soQ@mail.gmail.com>
Subject: Re: [PATCH 2/4] KVM: SVM: Refuse to load kvm_amd if NX support is not available
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jun 15, 2021 at 9:45 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Refuse to load KVM if NX support is not available.  Shadow paging has
> assumed NX support since commit 9167ab799362 ("KVM: vmx, svm: always run
> with EFER.NXE=1 when shadow paging is active"), and NPT has assumed NX
> support since commit b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation").
> While the NX huge pages mitigation should not be enabled by default for
> AMD CPUs, it can be turned on by userspace at will.
>
> Unlike Intel CPUs, AMD does not provide a way for firmware to disable NX
> support, and Linux always sets EFER.NX=1 if it is supported.  Given that
> it's extremely unlikely that a CPU supports NPT but not NX, making NX a
> formal requirement is far simpler than adding requirements to the
> mitigation flow.
>
> Fixes: 9167ab799362 ("KVM: vmx, svm: always run with EFER.NXE=1 when shadow paging is active")
> Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>