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Sat, 17 May 2025 05:43:29 -0700 (PDT) Received: from [192.168.21.214] ([69.164.134.123]) by smtp.googlemail.com with ESMTPSA id af79cd13be357-7cd468b6d30sm253925285a.76.2025.05.17.05.43.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 17 May 2025 05:43:29 -0700 (PDT) Message-ID: Date: Sat, 17 May 2025 14:43:28 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/3] KVM: x86/mmu: Defer allocation of shadow MMU's hashed page list To: Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Vipin Sharma References: <20250516215422.2550669-1-seanjc@google.com> <20250516215422.2550669-4-seanjc@google.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: <20250516215422.2550669-4-seanjc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 5/16/25 23:54, Sean Christopherson wrote: > When the TDP MMU is enabled, i.e. when the shadow MMU isn't used until a > nested TDP VM is run, defer allocation of the array of hashed lists used > to track shadow MMU pages until the first shadow root is allocated. > > Setting the list outside of mmu_lock is safe, as concurrent readers must > hold mmu_lock in some capacity, shadow pages can only be added (or removed) > from the list when mmu_lock is held for write, and tasks that are creating > a shadow root are serialized by slots_arch_lock. I.e. it's impossible for > the list to become non-empty until all readers go away, and so readers are > guaranteed to see an empty list even if they make multiple calls to > kvm_get_mmu_page_hash() in a single mmu_lock critical section. > > Use {WRITE/READ}_ONCE to set/get the list when mmu_lock isn't held for > write, out of an abundance of paranoia; no sane compiler should tear the > store or load, but it's technically possible. > > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/mmu/mmu.c | 60 +++++++++++++++++++++++++++++++++++------- > 1 file changed, 50 insertions(+), 10 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 41da2cb1e3f1..edb4ecff9917 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -1983,14 +1983,33 @@ static bool sp_has_gptes(struct kvm_mmu_page *sp) > return true; > } > > +static __ro_after_init HLIST_HEAD(empty_page_hash); > + > +static struct hlist_head *kvm_get_mmu_page_hash(struct kvm *kvm, gfn_t gfn) > +{ > + /* > + * Load mmu_page_hash from memory exactly once, as it's set at runtime > + * outside of mmu_lock when the TDP MMU is enabled, i.e. when the hash > + * table of shadow pages isn't needed unless KVM needs to shadow L1's > + * TDP for an L2 guest. Avoid double negations, for example "i.e. when shadow paging and the mmu_page_hash is only needed for a L1 hypervisor's TDP". > + */ > + struct hlist_head *page_hash = READ_ONCE(kvm->arch.mmu_page_hash); This READ_ONCE is more for the (now implicit) smp_read_barrier_depends() than for "loading exactly once". Might as well use smp_load_acquire() since it's free on x86. > + lockdep_assert_held(&kvm->mmu_lock); Move this comment here from kvm_mmu_alloc_page_hash(): /* mmu_lock must be held for write to add (or remove) shadow * pages, and so readers are guaranteed to see an empty list for * their current mmu_lock critical section. */ > + if (!page_hash) > + return &empty_page_hash; > + > + return &page_hash[kvm_page_table_hashfn(gfn)]; > +} > + > #define for_each_valid_sp(_kvm, _sp, _list) \ > hlist_for_each_entry(_sp, _list, hash_link) \ > if (is_obsolete_sp((_kvm), (_sp))) { \ > } else > > #define for_each_gfn_valid_sp_with_gptes(_kvm, _sp, _gfn) \ > - for_each_valid_sp(_kvm, _sp, \ > - &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)]) \ > + for_each_valid_sp(_kvm, _sp, kvm_get_mmu_page_hash(_kvm, _gfn)) \ > if ((_sp)->gfn != (_gfn) || !sp_has_gptes(_sp)) {} else > > static bool kvm_sync_page_check(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp) > @@ -2358,6 +2377,12 @@ static struct kvm_mmu_page *__kvm_mmu_get_shadow_page(struct kvm *kvm, > struct kvm_mmu_page *sp; > bool created = false; > > + /* > + * No need for READ_ONCE(), unlike in kvm_get_mmu_page_hash(), because > + * mmu_page_hash must be set prior to creating the first shadow root, > + * i.e. reaching this point is fully serialized by slots_arch_lock. > + */ > + BUG_ON(!kvm->arch.mmu_page_hash); > sp_list = &kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)]; > > sp = kvm_mmu_find_shadow_page(kvm, vcpu, gfn, sp_list, role); > @@ -3886,11 +3911,21 @@ static int kvm_mmu_alloc_page_hash(struct kvm *kvm) > { > typeof(kvm->arch.mmu_page_hash) h; > > + if (kvm->arch.mmu_page_hash) > + return 0; > + > h = kvcalloc(KVM_NUM_MMU_PAGES, sizeof(*h), GFP_KERNEL_ACCOUNT); > if (!h) > return -ENOMEM; > > - kvm->arch.mmu_page_hash = h; > + /* > + * Write mmu_page_hash exactly once as there may be concurrent readers, > + * e.g. to check for shadowed PTEs in mmu_try_to_unsync_pages(). Note, > + * mmu_lock must be held for write to add (or remove) shadow pages, and > + * so readers are guaranteed to see an empty list for their current > + * mmu_lock critical section. > + */ > + WRITE_ONCE(kvm->arch.mmu_page_hash, h); Use smp_store_release here (unlike READ_ONCE(), it's technically incorrect to use WRITE_ONCE() here!), with a remark that it pairs with kvm_get_mmu_page_hash(). That's both more accurate and leads to a better comment than "write exactly once". Paolo