[PATCH RFC v4 05/44] KVM: guest_memfd: Wire up kvm_get_memory_attributes() to per-gmem attributes

Ackerley Tng
2026-03-26T22:24:41Z

From: Sean Christopherson <seanjc@google.com>

Implement kvm_gmem_get_memory_attributes() for guest_memfd to allow the KVM
core and architecture code to query per-GFN memory attributes.

kvm_gmem_get_memory_attributes() finds the memory slot for a given GFN and
queries the guest_memfd file's to determine if the page is marked as
private.

If vm_memory_attributes is not enabled, there is no shared/private tracking
at the VM level. Install the guest_memfd implementation as long as
guest_memfd is enabled to give guest_memfd a chance to respond on
attributes.

guest_memfd should look up attributes regardless of whether this memslot is
gmem-only since attributes are now tracked by gmem regardless of whether
mmap() is enabled.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
include/linux/kvm_host.h | 2 ++
virt/kvm/guest_memfd.c | 37 +++++++++++++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 3 +++
3 files changed, 42 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 5393854ca5966..0f84421b3f49e 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2542,6 +2542,8 @@ bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
struct kvm_gfn_range *range);
#endif /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */

+unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn);
+
#ifdef CONFIG_KVM_GUEST_MEMFD
int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index aa2caf5114da2..6f4252c8b3f59 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -515,6 +515,43 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
return 0;
}

+unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
+{
+ struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+ struct inode *inode;
+ unsigned long attrs;
+
+ /*
+ * If this gfn has no associated memslot, there's no chance of the gfn
+ * being backed by private memory, since guest_memfd must be used for
+ * private memory, and guest_memfd must be associated with some memslot.
+ */
+ if (!slot)
+ return 0;
+
+ CLASS(gmem_get_file, file)(slot);
+ if (!file)
+ return 0;
+
+ inode = file_inode(file);
+
+ /*
+ * Acquire the filemap lock to ensure the mtree lookup gets a
+ * stable result. The caller _must_ still protect consumption
+ * of private vs. shared by checking
+ * mmu_invalidate_retry_gfn() under mmu_lock to serialize
+ * against ongoing attribute updates. Acquiring the filemap
+ * lock only ensures a stable _lookup_, the result can become
+ * stale as soon as the lock is dropped.
+ */
+ filemap_invalidate_lock_shared(inode->i_mapping);
+ attrs = kvm_gmem_get_attributes(inode, kvm_gmem_get_index(slot, gfn));
+ filemap_invalidate_unlock_shared(inode->i_mapping);
+
+ return attrs;
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_memory_attributes);
+
static struct file_operations kvm_gmem_fops = {
.mmap = kvm_gmem_mmap,
.open = generic_file_open,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dbdcef90a4950..7bb59679c9316 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2660,6 +2660,9 @@ static void kvm_init_memory_attributes(void)
if (vm_memory_attributes)
static_call_update(__kvm_get_memory_attributes,
kvm_get_vm_memory_attributes);
+ else if (IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
+ static_call_update(__kvm_get_memory_attributes,
+ kvm_gmem_get_memory_attributes);
else
static_call_update(__kvm_get_memory_attributes,
(void *)__static_call_return0);

--
2.53.0.1018.g2bb0e51243-goog