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AJvYcCVM2K0gd5hz2TSWH7gvuKRtjusAMhrB8cG4bpglp/cc+DLXcCSnwCQya9IZJQQ0NsKkPazNlIYLBN4=@vger.kernel.org X-Gm-Message-State: AOJu0YzX8GK1L93ba0lePmPhQkzSadJnzcVOXHx6REwedrZO4QuBxqbb Rlp1ui4X6DhTS/61vxvhu6pSwRY/WT38YXY40gVeA3+nxJemcaKhVUSj7KpY4MdVsUoBiOB4r9F trFxfKn+xYiegXOp0DQWnBaSBoA== X-Google-Smtp-Source: AGHT+IEPuBDuGNsrppGmcrNT5qFipxZMZW83jV/EN44QaRv4+wQ0X1ZUm0/OOFjwazq9d0SG+iIXrpEvGj5fqDDiKg== X-Received: from pjn8.prod.google.com ([2002:a17:90b:5708:b0:31f:2a78:943]) (user=ackerleytng job=prod-delivery.src-stubby-dispatcher) by 2002:a17:902:ea01:b0:267:d2f9:2327 with SMTP id d9443c01a7336-290c9cf2d88mr421610105ad.27.1761324115016; Fri, 24 Oct 2025 09:41:55 -0700 (PDT) Date: Fri, 24 Oct 2025 09:41:53 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <8ee16fbf254115b0fd72cc2b5c06d2ccef66eca9.1760731772.git.ackerleytng@google.com> <2457cb3b-5dde-4ca1-b75d-174b5daee28a@arm.com> Message-ID: Subject: Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2 From: Ackerley Tng To: Sean Christopherson Cc: Steven Price , cgroups@vger.kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, x86@kernel.org, akpm@linux-foundation.org, binbin.wu@linux.intel.com, bp@alien8.de, brauner@kernel.org, chao.p.peng@intel.com, chenhuacai@kernel.org, corbet@lwn.net, dave.hansen@intel.com, dave.hansen@linux.intel.com, david@redhat.com, dmatlack@google.com, erdemaktas@google.com, fan.du@intel.com, fvdl@google.com, haibo1.xu@intel.com, hannes@cmpxchg.org, hch@infradead.org, hpa@zytor.com, hughd@google.com, ira.weiny@intel.com, isaku.yamahata@intel.com, jack@suse.cz, james.morse@arm.com, jarkko@kernel.org, jgg@ziepe.ca, jgowans@amazon.com, jhubbard@nvidia.com, jthoughton@google.com, jun.miao@intel.com, kai.huang@intel.com, keirf@google.com, kent.overstreet@linux.dev, liam.merwick@oracle.com, maciej.wieczor-retman@intel.com, mail@maciej.szmigiero.name, maobibo@loongson.cn, mathieu.desnoyers@efficios.com, maz@kernel.org, mhiramat@kernel.org, mhocko@kernel.org, mic@digikod.net, michael.roth@amd.com, mingo@redhat.com, mlevitsk@redhat.com, mpe@ellerman.id.au, muchun.song@linux.dev, nikunj@amd.com, nsaenz@amazon.es, oliver.upton@linux.dev, palmer@dabbelt.com, pankaj.gupta@amd.com, paul.walmsley@sifive.com, pbonzini@redhat.com, peterx@redhat.com, pgonda@google.com, prsampat@amd.com, pvorel@suse.cz, qperret@google.com, richard.weiyang@gmail.com, rick.p.edgecombe@intel.com, rientjes@google.com, rostedt@goodmis.org, roypat@amazon.co.uk, rppt@kernel.org, shakeel.butt@linux.dev, shuah@kernel.org, suzuki.poulose@arm.com, tabba@google.com, tglx@linutronix.de, thomas.lendacky@amd.com, vannapurve@google.com, vbabka@suse.cz, viro@zeniv.linux.org.uk, vkuznets@redhat.com, will@kernel.org, willy@infradead.org, wyihan@google.com, xiaoyao.li@intel.com, yan.y.zhao@intel.com, yilun.xu@intel.com, yuzenghui@huawei.com Content-Type: text/plain; charset="UTF-8" Sean Christopherson writes: > On Fri, Oct 24, 2025, Ackerley Tng wrote: >> Sean Christopherson writes: >> >> >> >> [...snip...] >> >> >> >> I've been thinking more about this: >> >> #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES >> case KVM_CAP_MEMORY_ATTRIBUTES2: >> case KVM_CAP_MEMORY_ATTRIBUTES: >> if (!vm_memory_attributes) >> return 0; >> >> return kvm_supported_mem_attributes(kvm); >> #endif >> >> And the purpose of adding KVM_CAP_MEMORY_ATTRIBUTES2 is that >> KVM_CAP_MEMORY_ATTRIBUTES2 tells userspace that >> KVM_SET_MEMORY_ATTRIBUTES2 is available iff there are valid >> attributes. >> >> (So there's still a purpose) >> >> Without valid attributes, userspace can't tell if it should use >> KVM_SET_MEMORY_ATTRIBUTES or the 2 version. > > To do what? If there are no attributes, userspace can't do anything useful anyways. > >> I also added KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES, which tells >> userspace the valid attributes when calling KVM_SET_MEMORY_ATTRIBUTES2 >> on a guest_memfd: > > Ya, and that KVM_SET_MEMORY_ATTRIBUTES2 is supported on guest_memfd. > >> #ifdef CONFIG_KVM_GUEST_MEMFD >> case KVM_CAP_GUEST_MEMFD: >> return 1; >> case KVM_CAP_GUEST_MEMFD_FLAGS: >> return kvm_gmem_get_supported_flags(kvm); >> case KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES: >> if (vm_memory_attributes) >> return 0; >> >> return kvm_supported_mem_attributes(kvm); >> #endif >> >> So to set memory attributes, userspace should > > Userspace *can*. User could also decide it only wants to support guest_memfd > attributes, e.g. because the platform admins controls the entire stack and built > their entire operation around in-place conversion. > >> if (kvm_check_cap(KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES) > 0) >> use KVM_SET_MEMORY_ATTRIBUTES2 with guest_memfd >> else if (kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES2) > 0) >> use KVM_SET_MEMORY_ATTRIBUTES2 with VM fd >> else if (kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES) > 0) >> use KVM_SET_MEMORY_ATTRIBUTES with VM fd >> else >> can't set memory attributes >> >> Something like that? > > More or else, ya. > >> In selftests there's this, when KVM_SET_USER_MEMORY_REGION2 was >> introduced: >> >> #define TEST_REQUIRE_SET_USER_MEMORY_REGION2() \ >> __TEST_REQUIRE(kvm_has_cap(KVM_CAP_USER_MEMORY2), \ >> "KVM selftests now require KVM_SET_USER_MEMORY_REGION2 (introduced in v6.8)") >> >> But looks like there's no direct equivalent for the introduction of >> KVM_SET_MEMORY_ATTRIBUTES2? > > KVM_CAP_USER_MEMORY2 is the equivalent. > > There's was no need to enumerate anything beyond yes/no, because > SET_USER_MEMORY_REGION2 didn't introduce new flags, it expanded the size of the > structure passed in from userspace so that KVM_CAP_GUEST_MEMFD could be introduced > without breaking backwards compatibility. > >> The closest would be to add a TEST_REQUIRE_VALID_ATTRIBUTES() which >> checks KVM_CAP_MEMORY_ATTRIBUTES2 or >> KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES before making the vm or >> guest_memfd ioctl respsectively. > > Yes. This is what I did in my (never posted, but functional) version: > > @@ -486,6 +488,7 @@ struct kvm_vm *__vm_create(struct vm_shape shape, uint32_t nr_runnable_vcpus, > } > guest_rng = new_guest_random_state(guest_random_seed); > sync_global_to_guest(vm, guest_rng); > + sync_global_to_guest(vm, kvm_has_gmem_attributes); I ported this [1] except for syncing this value to the guest, because I think the guest shouldn't need to know this information, the host should decide what to do. I think, if the guests really need to know this, the test itself can do the syncing. [1] https://lore.kernel.org/all/5656d432df1217c08da0cc2694fd79948bfd686f.1760731772.git.ackerleytng@google.com/ > > kvm_arch_vm_post_create(vm, nr_runnable_vcpus); > > @@ -2319,6 +2333,8 @@ void __attribute((constructor)) kvm_selftest_init(void) > guest_random_seed = last_guest_seed = random(); > pr_info("Random seed: 0x%x\n", guest_random_seed); > > + kvm_has_gmem_attributes = kvm_has_cap(KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES); > + > kvm_selftest_arch_init(); > } > > That way the core library code can pivot on gmem vs. VM attributes without having > to rely on tests to define anything. E.g. > > static inline void vm_mem_set_memory_attributes(struct kvm_vm *vm, uint64_t gpa, > uint64_t size, uint64_t attrs) > { > if (kvm_has_gmem_attributes) { > off_t fd_offset; > uint64_t len; > int fd; > > fd = kvm_gpa_to_guest_memfd(vm, gpa, &fd_offset, &len); > TEST_ASSERT(len >= size, "Setting attributes beyond the length of a guest_memfd"); > gmem_set_memory_attributes(fd, fd_offset, size, attrs); > } else { > vm_set_memory_attributes(vm, gpa, size, attrs); > } > }