KVM: guest_memfd: Stub in ability to enable in-place shared<=>private conversion

Stub in global variable to enable in-place guest_memfd private<=>shared
memory conversion, which will eventually be exposed to userspace via a
module param, and wire up the __kvm_is_private_gfn() static call to the
guest_memfd version when in-place conversion is enabled, i.e. when gmem is
the sole authority on private vs. shared memory.

Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Fuad Tabba <tabba@google.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260910-gmem-inplace-conversion-v13-9-dd6fbf94f4e1@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Documentation/virt/kvm/api.rst

@@ -6381,11 +6381,16 @@ mapping for userspace_addr is not required to be valid/populated at the time of
 KVM_SET_USER_MEMORY_REGION2, e.g. shared memory can be lazily mapped/allocated
 on-demand.
 
-When mapping a gfn into the guest, KVM selects shared vs. private, i.e consumes
-userspace_addr vs. guest_memfd, based on the gfn's KVM_MEMORY_ATTRIBUTE_PRIVATE
-state.  At VM creation time, all memory is shared, i.e. the PRIVATE attribute
-is '0' for all gfns.  Userspace can control whether memory is shared/private by
-toggling KVM_MEMORY_ATTRIBUTE_PRIVATE via KVM_SET_MEMORY_ATTRIBUTES as needed.
+When mapping a gfn into the guest, KVM selects shared vs. private, i.e. consumes
+userspace_addr vs. guest_memfd, based on the state in guest_memfd, which is the
+sole authority on private vs. shared memory.  See :ref:`KVM_CREATE_GUEST_MEMFD`
+to find out more about the creation-time shared/private status.
+
+If in-place conversion is disabled, KVM selects shared vs. private based on the
+gfn's KVM_MEMORY_ATTRIBUTE_PRIVATE state.  At VM creation time, all memory is
+shared, i.e. the PRIVATE attribute is '0' for all gfns.  Userspace can control
+whether memory is shared/private by toggling KVM_MEMORY_ATTRIBUTE_PRIVATE via
+KVM_SET_MEMORY_ATTRIBUTES as needed.
 
 S390:
 ^^^^^
@@ -6429,6 +6434,8 @@ the state of a gfn/page as needed.
 
 The "flags" field is reserved for future extensions and must be '0'.
 
+.. _KVM_CREATE_GUEST_MEMFD:
+
 4.142 KVM_CREATE_GUEST_MEMFD
 ----------------------------
 

include/linux/kvm_host.h

@@ -2580,6 +2580,8 @@ static inline bool kvm_vm_is_private_gfn(struct kvm *kvm, gfn_t gfn)
 #endif  /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
 
 #ifdef kvm_arch_has_private_mem
+extern bool gmem_in_place_conversion;
+
 typedef bool (kvm_is_private_gfn_t)(struct kvm *kvm, gfn_t gfn);
 DECLARE_STATIC_CALL(__kvm_is_private_gfn, kvm_is_private_gfn_t);
 
@@ -2588,6 +2590,8 @@ static inline bool kvm_is_private_gfn(struct kvm *kvm, gfn_t gfn)
 	return static_call(__kvm_is_private_gfn)(kvm, gfn);
 }
 #else
+#define gmem_in_place_conversion false
+
 static inline bool kvm_is_private_gfn(struct kvm *kvm, gfn_t gfn)
 {
 	return false;
@@ -2595,6 +2599,8 @@ static inline bool kvm_is_private_gfn(struct kvm *kvm, gfn_t gfn)
 #endif /* kvm_arch_has_private_mem */
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
+bool kvm_gmem_is_private_gfn(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, int *max_order);
 #else

virt/kvm/guest_memfd.c

@@ -517,6 +517,32 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+bool kvm_gmem_is_private_gfn(struct kvm *kvm, gfn_t gfn)
+{
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+	struct inode *inode;
+
+	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_ protect consumption of private vs. shared either by
+	 * holding guest_memfd's invalidate lock for the entire duration, or by
+	 * checking mmu_invalidate_retry_gfn() under mmu_lock to serialize
+	 * against concurrent attribute updates.
+	 */
+	return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn));
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private_gfn);
+
 static struct file_operations kvm_gmem_fops = {
 	.mmap		= kvm_gmem_mmap,
 	.open		= generic_file_open,

virt/kvm/kvm_main.c

@@ -102,6 +102,10 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink);
 static bool __ro_after_init allow_unsafe_mappings;
 module_param(allow_unsafe_mappings, bool, 0444);
 
+#ifdef kvm_arch_has_private_mem
+bool __ro_after_init gmem_in_place_conversion = false;
+#endif
+
 /*
  * Ordering of locks:
  *
@@ -2442,6 +2446,9 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
 static u64 kvm_supported_vm_mem_attributes(struct kvm *kvm)
 {
 #ifdef kvm_arch_has_private_mem
+	if (gmem_in_place_conversion)
+		return 0;
+
 	if (!kvm || kvm_arch_has_private_mem(kvm))
 		return KVM_MEMORY_ATTRIBUTE_PRIVATE;
 #endif
@@ -2653,8 +2660,11 @@ EXPORT_STATIC_CALL_GPL(__kvm_is_private_gfn);
 
 static void kvm_init_memory_attributes(void)
 {
+	if (gmem_in_place_conversion)
+		static_call_update(__kvm_is_private_gfn, kvm_gmem_is_private_gfn);
 #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
-	static_call_update(__kvm_is_private_gfn, kvm_vm_is_private_gfn);
+	else
+		static_call_update(__kvm_is_private_gfn, kvm_vm_is_private_gfn);
 #endif
 }
 #else