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[54.240.197.234]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ebcb958sm23845574f8f.21.2026.10.27.08.46.57 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Oct 2026 08:46:59 -0700 (PDT) From: Fred Griffoul To: Ackerley Tng , Paolo Bonzini , Sean Christopherson , Jason Gunthorpe , Kevin Tian Cc: 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=20K=C3=B6nig?= , Connor Williamson , Dave Hansen , Dave Jiang , David Hildenbrand , David Rientjes , David Woodhouse , Erdem Aktas , Frank van der Linden , Gregory Price , "H . Peter Anvin" , Hugh Dickins , Ingo Molnar , Jacob Pan , James Houghton , Jan Kara , Jianxiong Gao , Joerg Roedel , Joey Gouly , John Starks , Jonathan Corbet , Justin Stitt , Lorenzo Stoakes , Marc Zyngier , Michael Roth , Mike Rapoport , Nathan Chancellor , Nick Desaulniers , Nicolin Chen , Oliver Upton , Pratyush Yadav , Randy Dunlap , Robin Murphy , Samiullah Khawaja , 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 , 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, iommu@lists.linux.dev Subject: [RFC PATCH 03/14] KVM: guest_memfd: Allow other subsystems to map guest_memfd files Date: Tue, 27 Oct 2026 15:46:38 +0000 Message-ID: <20261027154649.91571-4-griffoul@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261027154649.91571-1-griffoul@gmail.com> References: <20261022-gmem-alternative-providers-v2-0-5dd1dc6c660a@google.com> <20261027154649.91571-1-griffoul@gmail.com> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Fred Griffoul A device assigned to a guest must map the guest's shared memory and lose a page whenever guest_memfd takes it away. iommufd cannot do that: it pins what it maps, which guest_memfd does not allow, and a device cannot fault. Add an export interface. A subsystem attaches to a guest_memfd file, looks up the frame behind each shared page without taking a reference, and gets an update() callback when what a lookup returns for a range may have changed. The old frames stay valid until update() returns, so a device that never faults can switch each mapping in one step. update() is called on conversions and memory failure; punch holes, PFN mode and pages that get memory follow. The functions are exported for symbol_get(), since KVM may be a module while the consumer is built in. Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Fred Griffoul --- arch/x86/kvm/Makefile | 3 +- include/linux/guest_memfd.h | 55 +++++++++++++ virt/kvm/guest_memfd.c | 149 ++++++++++++++++++++++++++++++++++++ 3 files changed, 206 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile index 5fd01bd19d1c..25fb3d6e2032 100644 --- a/arch/x86/kvm/Makefile +++ b/arch/x86/kvm/Makefile @@ -62,7 +62,8 @@ exports_grep_trailer := --include='*.[ch]' -nrw $(srctree)/virt/kvm $(srctree)/a -e kvm_write_track_add_gfn \ -e kvm_write_track_remove_gfn \ -e kvm_file_to_kvm_fn \ - -e kvm_gmem_revoke + -e kvm_gmem_revoke \ + -e kvm_gmem_export # Force grep to emit a goofy group separator that can in turn be replaced with # the above newline macro (newlines in Make are a nightmare). Note, grep only diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h index 9d8764b93af2..fdf7f8404d6c 100644 --- a/include/linux/guest_memfd.h +++ b/include/linux/guest_memfd.h @@ -3,6 +3,7 @@ #define _LINUX_GUEST_MEMFD_H #include +#include #include struct file; @@ -82,4 +83,58 @@ static inline int kvm_gmem_revoke(struct inode *inode, pgoff_t start, size_t nr_ } #endif +/* + * Mapping guest_memfd files in other subsystems. + * + * A subsystem that maps guest_memfd pages, such as iommufd, can neither pin + * nor fault them. It attaches to the file, looks up the frame behind each + * shared page without taking a reference, and guest_memfd calls its update() + * whenever what a lookup returns for a range may have changed: conversions and + * memory failure. update() makes the consumer's mappings of the range match + * what a lookup returns now: a frame, or a hole for a private page or a page + * without a frame. It may be called for pages that did not change, and for + * ranges that do not follow the blocks that lookups report: the consumer may + * then remap whole blocks, also outside the range, e.g. to merge them into + * larger mappings. A consumer that can fault may instead only remove its + * mappings of the range. + * + * Until update() returns, the frames it replaces stay valid, so a consumer + * may switch each mapping to its new target in one step; once it returns, + * the consumer no longer accesses them. update() cannot fail: a consumer + * that needs memory to apply one, e.g. to split a large mapping, reserves it + * beforehand. + * + * guest_memfd calls update() with its invalidate lock held, exclusive when + * pages convert and shared on memory failure, possibly with a folio of the + * file locked; so update() may only call kvm_gmem_export_lookup_pfn(). Under + * the shared lock, updates may run at the same time, also for overlapping + * ranges. The consumer holds a lock across a lookup and the mapping it + * installs, which update() also takes. Lock order: + * + * guest_memfd invalidate lock + * consumer lock + * + * A consumer attaches and detaches outside its lock. KVM may be a module + * while the consumer is built in: consumers reach these with symbol_get(). + */ +struct guest_memfd_export; + +struct guest_memfd_export_ops { + void (*update)(struct guest_memfd_export *e, pgoff_t start, + pgoff_t nr_pages); +}; + +struct guest_memfd_export { + const struct guest_memfd_export_ops *ops; + + /* Private to guest_memfd. */ + struct list_head node; + struct file *file; +}; + +int kvm_gmem_export_attach(struct file *file, struct guest_memfd_export *e); +void kvm_gmem_export_detach(struct guest_memfd_export *e); +int kvm_gmem_export_lookup_pfn(struct guest_memfd_export *e, pgoff_t index, + unsigned long *pfn, int *max_order, u32 *attrs); + #endif /* _LINUX_GUEST_MEMFD_H */ diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 1554a17906da..81031483a1f1 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -38,6 +38,8 @@ struct gmem_inode { void *provider; const struct guest_memfd_provider_operations *provider_ops; + /* Subsystems that map the file, see include/linux/guest_memfd.h. */ + struct list_head exports; u64 flags; /* @@ -420,6 +422,21 @@ static void kvm_gmem_invalidate_end(struct inode *inode, pgoff_t start, __kvm_gmem_invalidate_end(f, start, end); } +/* + * What a lookup returns for [start, end) may have changed: exclusive lock when + * pages converted, shared on memory failure. + */ +static void kvm_gmem_update_exports(struct inode *inode, pgoff_t start, + pgoff_t end) +{ + struct guest_memfd_export *e; + + lockdep_assert_held(&inode->i_mapping->invalidate_lock); + + list_for_each_entry(e, &GMEM_I(inode)->exports, node) + e->ops->update(e, start, end - start); +} + #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM static void kvm_gmem_pfn_reclaim(struct gmem_inode *gi, pgoff_t start, pgoff_t end) { @@ -1130,6 +1147,9 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, mas_store_prealloc(&mas, xa_mk_value(attrs)); + /* After the store, before a vCPU can fault a private page in. */ + kvm_gmem_update_exports(inode, start, end); + kvm_gmem_invalidate_end(inode, start, end); out: filemap_invalidate_unlock(mapping); @@ -1204,6 +1224,131 @@ static struct file_operations kvm_gmem_fops = { .unlocked_ioctl = kvm_gmem_ioctl, }; +/** + * kvm_gmem_export_attach - Map a guest_memfd file in another subsystem. + * @file: The guest_memfd file. + * @e: The export, with @e->ops set. + * + * guest_memfd calls @e->ops->update() from now on, and holds a reference to + * @file until kvm_gmem_export_detach(). May sleep. + * + * Return: 0, -ENODEV if @file is not a guest_memfd file, or -EOPNOTSUPP in + * PFN mode. + */ +int kvm_gmem_export_attach(struct file *file, struct guest_memfd_export *e) +{ + struct inode *inode = file_inode(file); + + if (file->f_op != &kvm_gmem_fops) + return -ENODEV; + if (kvm_gmem_is_pfn_mode(GMEM_I(inode))) + return -EOPNOTSUPP; + + e->file = get_file(file); + filemap_invalidate_lock(inode->i_mapping); + list_add_tail(&e->node, &GMEM_I(inode)->exports); + filemap_invalidate_unlock(inode->i_mapping); + return 0; +} +EXPORT_SYMBOL_GPL(kvm_gmem_export_attach); + +/** + * kvm_gmem_export_detach - Stop mapping a guest_memfd file. + * @e: The export. + * + * Waits for an update() in progress. May sleep. + */ +void kvm_gmem_export_detach(struct guest_memfd_export *e) +{ + struct inode *inode = file_inode(e->file); + + filemap_invalidate_lock(inode->i_mapping); + list_del(&e->node); + filemap_invalidate_unlock(inode->i_mapping); + fput(e->file); +} +EXPORT_SYMBOL_GPL(kvm_gmem_export_detach); + +/* Are [index, index + nr) all shared? Without the invalidate lock. */ +static bool kvm_gmem_range_is_shared(struct inode *inode, pgoff_t index, + unsigned long nr) +{ + unsigned long first = index; + void *entry; + + mt_for_each(&GMEM_I(inode)->attributes, entry, first, index + nr - 1) { + if (kvm_gmem_get_attributes(inode, entry) & + KVM_MEMORY_ATTRIBUTE_PRIVATE) + return false; + } + return true; +} + +/* What kvm_gmem_export_lookup_pfn() returns for a page inside the file. */ +static int __kvm_gmem_export_lookup(struct inode *inode, pgoff_t index, + unsigned long *pfn, int *max_order, + u32 *attrs) +{ + struct folio *folio; + + if (!kvm_gmem_range_is_shared(inode, index, 1)) + return -ENOENT; + + /* A folio that was never zeroed is a hole. */ + folio = filemap_get_folio(inode->i_mapping, index); + if (IS_ERR(folio)) + return -ENOENT; + if (folio_test_hwpoison(folio) || !folio_test_uptodate(folio)) { + folio_put(folio); + return -ENOENT; + } + *pfn = folio_file_pfn(folio, index); + *max_order = 0; + *attrs = 0; + folio_put(folio); + + /* Nor may the block hold a private page. */ + while (*max_order && + !kvm_gmem_range_is_shared(inode, + round_down(index, 1UL << *max_order), + 1UL << *max_order)) + (*max_order)--; + return 0; +} + +/** + * kvm_gmem_export_lookup_pfn - The frame behind a shared page. + * @e: The export. + * @index: The page index in the file. + * @pfn: The frame. + * @max_order: In: the largest order the caller can map. Out: the order of + * the naturally aligned block around @index that is contiguous, + * shared and of the same @attrs; 0 for -ENOENT. + * @attrs: The GUEST_MEMFD_PFN_* attributes of the block. + * + * Takes no reference and allocates nothing, and may be called from the + * export's update(): the answer holds until the update() that changes it. + * + * Context: May sleep. + * Return: 0, -ENOENT for a private page or a page without a frame, -EINVAL + * beyond the end of the file, or another error from the provider. + */ +int kvm_gmem_export_lookup_pfn(struct guest_memfd_export *e, pgoff_t index, + unsigned long *pfn, int *max_order, u32 *attrs) +{ + struct inode *inode = file_inode(e->file); + int r; + + if (index >= i_size_read(inode) >> PAGE_SHIFT) + return -EINVAL; + + r = __kvm_gmem_export_lookup(inode, index, pfn, max_order, attrs); + if (r == -ENOENT) + *max_order = 0; + return r; +} +EXPORT_SYMBOL_GPL(kvm_gmem_export_lookup_pfn); + static int kvm_gmem_migrate_folio(struct address_space *mapping, struct folio *dst, struct folio *src, enum migrate_mode mode) @@ -1241,6 +1386,9 @@ static int kvm_gmem_error_folio(struct address_space *mapping, struct folio *fol filter = kvm_gmem_get_all_gfns_filter(inode); kvm_gmem_invalidate_start(inode, start, end, filter); + /* Lookups see the poisoned folio as a hole: take it from the exports. */ + kvm_gmem_update_exports(inode, start, end); + /* * Do not truncate the range, what action is taken in response to the * error is userspace's decision (assuming the architecture supports @@ -1867,6 +2015,7 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb) gi->provider = NULL; gi->provider_ops = NULL; INIT_LIST_HEAD(&gi->gmem_file_list); + INIT_LIST_HEAD(&gi->exports); return &gi->vfs_inode; } -- 2.47.3