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Wed, 11 Jun 2025 11:21:22 -0700 (PDT) Received: from [192.168.10.81] ([151.49.64.79]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-4532518cc04sm28635245e9.25.2025.06.11.11.21.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 11 Jun 2025 11:21:21 -0700 (PDT) Message-ID: <5fee2f3b-03de-442b-acaf-4591638c8bb5@redhat.com> Date: Wed, 11 Jun 2025 20:21:16 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Paolo Bonzini Subject: Re: [PATCH] KVM: x86/mmu: Embed direct bits into gpa for KVM_PRE_FAULT_MEMORY To: Sean Christopherson Cc: Xiaoyao Li , rick.p.edgecombe@intel.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, yan.y.zhao@intel.com, reinette.chatre@intel.com, kai.huang@intel.com, adrian.hunter@intel.com, isaku.yamahata@intel.com, Binbin Wu , tony.lindgren@linux.intel.com References: <20250611001018.2179964-1-xiaoyao.li@intel.com> Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On Wed, Jun 11, 2025 at 8:10 PM Sean Christopherson wrote: > > + direct_bits = 0; > > if (kvm_arch_has_private_mem(vcpu->kvm) && > > kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(range->gpa))) > > error_code |= PFERR_PRIVATE_ACCESS; > > + else > > + direct_bits = gfn_to_gpa(kvm_gfn_direct_bits(vcpu->kvm)); > > Eww. It's bad enough that TDX bleeds it's mirror needs into common MMU code, > but stuffing vendor specific GPA bits in common code goes too far. Actually, > all of this goes too far. There's zero reason any code outside of TDX needs to > *explicitly* care whether mirrors or "direct" MMUs have mandatory gfn bits. > > Back to the main topic, KVM needs to have a single source of truth when it comes > to whether a fault is private and thus mirrored (or not). Common KVM needs to be > aware of aliased GFN bits, but absolute nothing outside of TDX (including common > VMX code) should be aware the mirror vs. "direct" (I hate that terminology; KVM > has far, far too much history and baggage with "direct") is tied to the existence > and polarity of aliased GFN bits. > > To detect a mirror fault: > > static inline bool kvm_is_mirror_fault(struct kvm *kvm, u64 error_code) > { > return kvm_has_mirrored_tdp(kvm) && > error_code & PFERR_PRIVATE_ACCESS; > } > > And for TDX, it should darn well explicitly track the shared GPA mask: > > static bool tdx_is_private_gpa(struct kvm *kvm, gpa_t gpa) > { > /* For TDX the direct mask is the shared mask. */ > return !(gpa & to_kvm_tdx(kvm)->shared_gpa_mask); > } My fault - this is more similar, at least in spirit, to what Yan and Xiaoyao had tested earlier: diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h index 52acf99d40a0..209103bf0f30 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.h +++ b/arch/x86/kvm/mmu/tdp_mmu.h @@ -48,7 +48,7 @@ static inline enum kvm_tdp_mmu_root_types static inline struct kvm_mmu_page *tdp_mmu_get_root_for_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) { - if (unlikely(!kvm_is_addr_direct(vcpu->kvm, fault->addr))) + if (unlikely(fault->is_private)) return root_to_sp(vcpu->arch.mmu->mirror_root_hpa); and I instead proposed the version that you hate with such ardor. My reasoning was that I preferred to have the pre-fault scenario "look like" what you get while the VM runs. > Outside of TDX, detecting mirrors, and anti-aliasing logic, the only use of > kvm_gfn_direct_bits() is to constrain TDP MMU walks to the appropriate gfn range. > And for that, we can simply use kvm_mmu_page.gfn, with a kvm_x86_ops hook to get > the TDP MMU root GFN (root allocation is a slow path, the CALL+RET is a non-issue). > > Compile tested only, and obviously needs to be split into multiple patches. Also obviously needs to be delayed to 6.17, since a working fix can be a one line change. :) (Plus your kvm_is_gfn_alias() test which should be included anyway and independently). What do you hate less between Yan's idea above and this patch? Just tell me and I'll handle posting v2. Paolo