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charset="utf-8" Message-Id: <20261022-gmem-alternative-providers-v2-5-5dd1dc6c660a@google.com> References: <20261022-gmem-alternative-providers-v2-0-5dd1dc6c660a@google.com> In-Reply-To: <20261022-gmem-alternative-providers-v2-0-5dd1dc6c660a@google.com> To: Alex Williamson , Alexander Graf , Alexander Viro , Alexey Kardashevskiy , Andrew Morton , "Aneesh Kumar K.V" , Ashish Kalra , Baolin Wang , Baolu Lu , Bill Wendling , Borislav Petkov , Catalin Marinas , Christian Brauner , =?utf-8?q?Christian_K=C3=B6nig?= , Connor Williamson , Dave Hansen , Dave Jiang , David Hildenbrand , David Rientjes , David Woodhouse , Erdem Aktas , Frank van der Linden , Fred Griffoul , Fred Griffoul , Gregory Price , "H. Peter Anvin" , Hugh Dickins , Ingo Molnar , Jacob Pan , James Houghton , Jan Kara , Jason Gunthorpe , Jianxiong Gao , Joerg Roedel , Joey Gouly , John Starks , Jonathan Corbet , Justin Stitt , Kevin Tian , Lorenzo Stoakes , Marc Zyngier , Michael Roth , Mike Rapoport , Nathan Chancellor , Nick Desaulniers , Nicolin Chen , Oliver Upton , Paolo Bonzini , Pratyush Yadav , Randy Dunlap , Robin Murphy , Samiullah Khawaja , Sean Christopherson , Shuah Khan , Steffen Eiden , Steven Price , Suzuki K Poulose , Tarun Sahu , Thomas Gleixner , Tom Lendacky , Tycho Andersen , Vishal Annapurve , Vlastimil Babka , Will Deacon , Xu Yilun , Yan Zhao , Zenghui Yu , x86@kernel.org, Shuah Khan , Fuad Tabba Cc: 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, llvm@lists.linux.dev, Ackerley Tng X-Mailer: b4 0.17-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1792711665; l=7993; i=ackerleytng@google.com; s=20260225; h=from:subject:message-id; bh=0G7kXeyhGqB7exCu+HDsg+EnEsp2/ypWTk3DC39+hvE=; b=fQD6dtNNlL2hF2L5aYCiKcASOb4iN3nVdhq4xDq+Zmvair/KrTP0zAbDvn3S8sRn6+9LtRAod l8RrBkymPLDCxtRQo3LbuNT74+rp+rPgvnx2NCD/ZHhrjzMflUcb60w X-Developer-Key: i=ackerleytng@google.com; a=ed25519; pk=sAZDYXdm6Iz8FHitpHeFlCMXwabodTm7p8/3/8xUxuU= X-Endpoint-Received: by B4 Relay for ackerleytng@google.com/20260225 with auth_id=649 X-Original-From: Ackerley Tng Reply-To: ackerleytng@google.com Content-Transfer-Encoding: 7bit From: Ackerley Tng A refcount on the resource_fd is taken only to keep the reference stable for guest_memfd to figure out the existence of a provider. The provider is responsible for pinning anything required to keep the provider's ops and the actual provided resource alive. .release() is called when this guest_memfd inode is destroyed to allow the provider to clean up. Signed-off-by: Ackerley Tng --- Documentation/virt/kvm/api.rst | 28 +++++++++++++++++---------- include/linux/kvm_host.h | 2 +- include/uapi/linux/kvm.h | 5 ++++- virt/kvm/guest_memfd.c | 44 ++++++++++++++++++++++++++++++++++-------- 4 files changed, 59 insertions(+), 20 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index e95c9cda085d9..9f5313d405d9c 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6486,7 +6486,9 @@ and cannot be resized (guest_memfd files do however support PUNCH_HOLE). struct kvm_create_guest_memfd { __u64 size; __u64 flags; - __u64 reserved[6]; + __s32 resource_fd; + __u32 pad; + __u64 reserved[5]; }; Conceptually, the inode backing a guest_memfd file represents physical memory, @@ -6508,15 +6510,21 @@ a single guest_memfd file, but the bound ranges must not overlap). The capability KVM_CAP_GUEST_MEMFD_FLAGS enumerates the `flags` that can be specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags: - ============================ ================================================ - GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file - descriptor. - GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during - KVM_CREATE_GUEST_MEMFD (memory files created - without INIT_SHARED will be marked private). - Shared memory can be faulted into host userspace - page tables. Private memory cannot. - ============================ ================================================ + ============================= ================================================ + GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file + descriptor. + GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during + KVM_CREATE_GUEST_MEMFD (memory files created + without INIT_SHARED will be marked private). + Shared memory can be faulted into host userspace + page tables. Private memory cannot. + GUEST_MEMFD_FLAG_USE_RESOURCE Consume resource_fd as an external memory + provider or resource pool. When not set, + resource_fd must be '0'. When set, the provider + must be a supported resource (e.g. tmpfs mount + root directory). Currently, tmpfs resource pools + require noswap and huge=never mount options. + ============================= ================================================ When the KVM MMU performs a PFN lookup to service a guest fault, the fault will always be consumed from guest_memfd, regardless of whether it is a shared or a diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index e7fc94e2910a8..d820afac49252 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -2618,7 +2618,7 @@ bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm); static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm) { - u64 flags = GUEST_MEMFD_FLAG_MMAP; + u64 flags = GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_USE_RESOURCE; if (!kvm || kvm_arch_supports_gmem_init_shared(kvm)) flags |= GUEST_MEMFD_FLAG_INIT_SHARED; diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index 8dff2fc1972e9..f4108bf7e3192 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -1674,11 +1674,14 @@ struct kvm_memory_attributes2 { #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd) #define GUEST_MEMFD_FLAG_MMAP (1ULL << 0) #define GUEST_MEMFD_FLAG_INIT_SHARED (1ULL << 1) +#define GUEST_MEMFD_FLAG_USE_RESOURCE (1ULL << 2) struct kvm_create_guest_memfd { __u64 size; __u64 flags; - __u64 reserved[6]; + __s32 resource_fd; + __u32 pad; + __u64 reserved[5]; }; #define KVM_PRE_FAULT_MEMORY _IOWR(KVMIO, 0xd5, struct kvm_pre_fault_memory) diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index ee823f2ef0368..84877beed6701 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -85,15 +85,26 @@ static const struct guest_memfd_provider_operations gmem_native_provider_ops = { static int kvm_gmem_attach_resource(struct inode *inode, int resource_fd) { const struct guest_memfd_provider_operations *ops = NULL; + void *provider = ERR_PTR(-EOPNOTSUPP); struct file *resource_file = NULL; - void *provider; - if (resource_fd == GMEM_RESOURCE_FD_NATIVE) + if (resource_fd == GMEM_RESOURCE_FD_NATIVE) { ops = &gmem_native_provider_ops; - else - return -EOPNOTSUPP; + } else { + resource_file = fget_raw(resource_fd); + if (!resource_file) + return -EBADF; + + if (resource_file->f_op) + ops = resource_file->f_op->gmem_provider_ops; + } + + if (ops && ops->attach && ops->alloc_folio) + provider = ops->attach(inode, resource_file); + + if (resource_file) + fput(resource_file); - provider = ops->attach(inode, resource_file); if (IS_ERR(provider)) return PTR_ERR(provider); @@ -120,6 +131,12 @@ static void gmem_provider_invalidate_folio(struct gmem_inode *gi, gi->provider_ops->invalidate_folio(gi->provider, folio); } +static void gmem_provider_release(struct gmem_inode *gi) +{ + if (gi->provider_ops && gi->provider_ops->release) + gi->provider_ops->release(gi->provider); +} + #define kvm_gmem_for_each_file(f, inode) \ list_for_each_entry(f, &GMEM_I(inode)->gmem_file_list, entry) @@ -1033,7 +1050,7 @@ static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags) return r; } -static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) +static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags, int resource_fd) { static const char *name = "[kvm-gmem]"; struct gmem_file *f; @@ -1088,7 +1105,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) */ smp_wmb(); - err = kvm_gmem_attach_resource(inode, GMEM_RESOURCE_FD_NATIVE); + err = kvm_gmem_attach_resource(inode, resource_fd); if (err) { fput(file); put_unused_fd(fd); @@ -1111,6 +1128,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args) { + int resource_fd = GMEM_RESOURCE_FD_NATIVE; loff_t size = args->size; u64 flags = args->flags; @@ -1120,7 +1138,15 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args) if (size <= 0 || !PAGE_ALIGNED(size)) return -EINVAL; - return __kvm_gmem_create(kvm, size, flags); + if (flags & GUEST_MEMFD_FLAG_USE_RESOURCE) { + if (args->resource_fd < 0) + return -EBADF; + resource_fd = args->resource_fd; + } else if (args->resource_fd) { + return -EINVAL; + } + + return __kvm_gmem_create(kvm, size, flags, resource_fd); } int kvm_gmem_prepare_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot, @@ -1464,6 +1490,8 @@ static void kvm_gmem_destroy_inode(struct inode *inode) { struct gmem_inode *gi = GMEM_I(inode); + gmem_provider_release(gi); + mpol_free_shared_policy(&gi->policy); /* -- 2.56.0.257.gf7497c7d27-goog