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>

2 files changed, 37 insertions, 1 deletions

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 3b05c6443f0f..134fed0e24fa 100644
--- a/virt/kvm/guest_memfd.c
+++ b/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,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 19098a51c35d..2532d1252a10 100644
--- a/virt/kvm/kvm_main.c
+++ b/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