[PATCH v10 09/16] KVM: guest_memfd: Track shared memory support in memslot

[Fuad Tabba <tabba@xxxxxxxxxx>]

Track whether a guest_memfd-backed memslot supports shared memory within
the memslot itself, using the flags field. The top half of memslot flags
is reserved for internal use in KVM. Add a flag there to track shared
memory support.

This saves the caller from having to check the guest_memfd-backed file
for this support, a potentially more expensive operation due to the need
to get/put the file.

Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
 include/linux/kvm_host.h | 11 ++++++++++-
 virt/kvm/guest_memfd.c   |  8 ++++++--
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ba83547e62b0..edb3795a64b9 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -54,7 +54,8 @@
  * used in kvm, other bits are visible for userspace which are defined in
  * include/uapi/linux/kvm.h.
  */
-#define KVM_MEMSLOT_INVALID	(1UL << 16)
+#define KVM_MEMSLOT_INVALID		(1UL << 16)
+#define KVM_MEMSLOT_SUPPORTS_SHARED	(1UL << 17)
 
 /*
  * Bit 63 of the memslot generation number is an "update in-progress flag",
@@ -2502,6 +2503,14 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
 		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
 }
 
+static inline bool kvm_gmem_memslot_supports_shared(const struct kvm_memory_slot *slot)
+{
+	if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
+		return false;
+
+	return slot->flags & KVM_MEMSLOT_SUPPORTS_SHARED;
+}
+
 #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
 static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
 {
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 5d34712f64fc..9ded8d5139ee 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -555,6 +555,7 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	loff_t size = slot->npages << PAGE_SHIFT;
 	unsigned long start, end;
 	struct kvm_gmem *gmem;
+	bool supports_shared;
 	struct inode *inode;
 	struct file *file;
 	int r = -EINVAL;
@@ -578,8 +579,9 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	    offset + size > i_size_read(inode))
 		goto err;
 
-	if (kvm_gmem_supports_shared(inode) &&
-	    !kvm_arch_supports_gmem_shared_mem(kvm))
+	supports_shared = kvm_gmem_supports_shared(inode);
+
+	if (supports_shared && !kvm_arch_supports_gmem_shared_mem(kvm))
 		goto err;
 
 	filemap_invalidate_lock(inode->i_mapping);
@@ -600,6 +602,8 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	 */
 	WRITE_ONCE(slot->gmem.file, file);
 	slot->gmem.pgoff = start;
+	if (supports_shared)
+		slot->flags |= KVM_MEMSLOT_SUPPORTS_SHARED;
 
 	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
 	filemap_invalidate_unlock(inode->i_mapping);
-- 
2.49.0.1164.gab81da1b16-goog