[PATCH v9 06/41] KVM: guest_memfd: Introduce function to check GFN private/shared status

[Ackerley Tng via B4 Relay]

From: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Introduce a function for KVM to check the private/shared status of guest
memory at a given GFN.

This will be used in a later patch.

Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
Tested-by: Shivank Garg <shivankg@xxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 include/linux/kvm_host.h |  2 ++
 virt/kvm/guest_memfd.c   | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 438567e3fa6a1..73f564b87a324 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2594,6 +2594,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 #endif /* kvm_arch_has_private_mem */
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
+bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn);
+
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
 		     int *max_order);
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 8c7709a352cc7..33c9830190e2e 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -474,6 +474,38 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn)
+{
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+	struct inode *inode;
+
+	/*
+	 * guest_memfd is the only provider of private memory and
+	 * guest_memfd must be used with a memslot, hence if there's
+	 * no associated memslot, there's no chance of this gfn being
+	 * private.
+	 */
+	if (!slot)
+		return 0;
+
+	CLASS(gmem_get_file, file)(slot);
+	if (!file)
+		return 0;
+
+	inode = file_inode(file);
+
+	/*
+	 * Rely on the maple tree's internal RCU lock to ensure a
+	 * stable result. This result can become stale as soon as the
+	 * lock is dropped, so 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.
+	 */
+	return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn));
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private);
+
 static struct file_operations kvm_gmem_fops = {
 	.mmap		= kvm_gmem_mmap,
 	.open		= generic_file_open,

-- 
2.55.0.508.g3f0d502094-goog