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AJvYcCWotzzQV8b2Rvoi69QfPADhAY1xXX5HrWNmcWQAnKwzg4L2EQBYGpD5ZDmcg4mF5c2sy8zLKgeI6Thz@lists.linux.dev X-Gm-Message-State: AOJu0Yx5/RIsuPlBFardGyuXOuP8KPa59ItcYQKDLXMN4JoRlJfqeNal McsI+QRbJZqttJV36fnXH8CzmaDMEd53jXD3ZgJ7IFgnK0ATFL2mzS8ISvCedyB2tOkKT45UP56 JCCT0qQ== X-Received: from pjd11.prod.google.com ([2002:a17:90b:54cb:b0:353:28b4:c297]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90a:d445:b0:340:e8e9:cc76 with SMTP id 98e67ed59e1d1-3543b309885mr823538a91.11.1769725404404; Thu, 29 Jan 2026 14:23:24 -0800 (PST) Date: Thu, 29 Jan 2026 14:23:23 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260129011517.3545883-1-seanjc@google.com> <20260129011517.3545883-3-seanjc@google.com> Message-ID: Subject: Re: [RFC PATCH v5 02/45] KVM: x86/mmu: Update iter->old_spte if cmpxchg64 on mirror SPTE "fails" From: Sean Christopherson To: Rick P Edgecombe Cc: "x86@kernel.org" , "dave.hansen@linux.intel.com" , "kas@kernel.org" , "bp@alien8.de" , "mingo@redhat.com" , "pbonzini@redhat.com" , "tglx@kernel.org" , Kai Huang , "ackerleytng@google.com" , "sagis@google.com" , Vishal Annapurve , "linux-kernel@vger.kernel.org" , Yan Y Zhao , Xiaoyao Li , "kvm@vger.kernel.org" , "linux-coco@lists.linux.dev" , Isaku Yamahata , "binbin.wu@linux.intel.com" Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Thu, Jan 29, 2026, Rick P Edgecombe wrote: > On Wed, 2026-01-28 at 17:14 -0800, Sean Christopherson wrote: > > Pass a pointer to iter->old_spte, not simply its value, when setting an > > external SPTE in __tdp_mmu_set_spte_atomic(), so that the iterator's va= lue > > will be updated if the cmpxchg64 to freeze the mirror SPTE fails. > >=20 >=20 > Might be being dense here, but is the bug that if cmpxchg64 *succeeds* an= d > set_external_spte() fails? Then old_spte is not updated and the local ret= ry will > expect the wrong old_spte. No, the bug is if the cmpxchg64 fails. On failure, the current mismatching= value is stored in the "old" param. KVM relies on the iter->old_spte holding the current value when restarting an operation without re-reading the SPTE from= memory. E.g. in __tdp_mmu_zap_root(), if tdp_mmu_set_spte_atomic() fails, iter->old= _spte *must* hold the current in-memroy value, otherwise the loop will hang becau= se it will re-attempt cmpxchg64 using the stale iter->old_spte. static void __tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root, bool shared, int zap_level) { struct tdp_iter iter; for_each_tdp_pte_min_level_all(iter, root, zap_level) { retry: if (tdp_mmu_iter_cond_resched(kvm, &iter, false, shared)) continue; if (!is_shadow_present_pte(iter.old_spte)) continue; if (iter.level > zap_level) continue; if (!shared) tdp_mmu_iter_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE); else if (tdp_mmu_set_spte_atomic(kvm, &iter, SHADOW_NONPRESENT_VALUE)) goto retry; } } > > =C2=A0 The bug > > is currently benign as TDX is mutualy exclusive with all paths that do > > "local" retry", e.g. clear_dirty_gfn_range() and wrprot_gfn_range(). >=20 >=20