Currently QEMU supports using guest_memfd internally (separately from
user-specified memory backends) to handle private memory for
confidential VMs. While KVM can switch between guest_memfd-backed
private memory and non-guest_memfd-backed shared memory via
KVM_SET_MEMORY_ATTRIBUTES, the memory in the guest_memfd inode can only
ever be private memory.

This is distinct from upcoming in-place conversion support, where
guest_memfd inodes can contain both private/shared memory and can
convert between the 2 in-place.

To help distinguish between these 2 uses of guest_memfd, add a dedicated
helper to handle the private-only uses of guest_memfd, and add some
additional sanity checks with that use-case in mind.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 accel/kvm/kvm-all.c    | 15 +++++++++++++++
 accel/stubs/kvm-stub.c |  6 ++++++
 include/system/kvm.h   |  1 +
 system/physmem.c       |  6 +++---
 4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 518baa35b9..15f237de4e 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -794,6 +794,11 @@ static int kvm_mem_flags(MemoryRegion *mr)
     }
     if (memory_region_has_guest_memfd_private(mr)) {
         assert(kvm_guest_memfd_supported);
+        /*
+         * memory_region_has_guest_memfd_private() is specifically pertaining to
+         * using guest_memfd to handle private memory use cases.
+         */
+        assert(kvm_supported_memory_attributes & KVM_MEMORY_ATTRIBUTE_PRIVATE);
         flags |= KVM_MEM_GUEST_MEMFD;
     }
     return flags;
@@ -4873,3 +4878,13 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
 
     return fd;
 }
+
+int kvm_create_guest_memfd_private(uint64_t size, Error **errp)
+{
+    if (!(kvm_supported_memory_attributes & KVM_MEMORY_ATTRIBUTE_PRIVATE)) {
+        error_setg(errp, "KVM does not support using guest_memfd for private memory");
+        return -1;
+    }
+
+    return kvm_create_guest_memfd(size, 0, errp);
+}
diff --git a/accel/stubs/kvm-stub.c b/accel/stubs/kvm-stub.c
index acbd0785e0..9fe58efe91 100644
--- a/accel/stubs/kvm-stub.c
+++ b/accel/stubs/kvm-stub.c
@@ -145,6 +145,12 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
     return -ENOSYS;
 }
 
+int kvm_create_guest_memfd_private(uint64_t size, Error **errp)
+{
+    error_setg(errp, "KVM is not enabled");
+    return -ENOSYS;
+}
+
 bool kvm_private_memory_attribute_supported(void)
 {
     return false;
diff --git a/include/system/kvm.h b/include/system/kvm.h
index d29624034c..b1e43ddc93 100644
--- a/include/system/kvm.h
+++ b/include/system/kvm.h
@@ -548,6 +548,7 @@ void kvm_mark_guest_state_protected(void);
 bool kvm_hwpoisoned_mem(void);
 
 int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp);
+int kvm_create_guest_memfd_private(uint64_t size, Error **errp);
 
 int kvm_set_memory_attributes_private(hwaddr start, uint64_t size);
 int kvm_set_memory_attributes_shared(hwaddr start, uint64_t size);
diff --git a/system/physmem.c b/system/physmem.c
index f6dff18bbb..991f7bb815 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2209,7 +2209,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
         }
 
         new_block->guest_memfd_private =
-            kvm_create_guest_memfd(new_block->max_length, 0, errp);
+            kvm_create_guest_memfd_private(new_block->max_length, errp);
         if (new_block->guest_memfd_private < 0) {
             qemu_mutex_unlock_ramlist();
             goto out_free;
@@ -2839,8 +2839,8 @@ int ram_block_rebind(Error **errp)
             if (block->guest_memfd_private >= 0) {
                 close(block->guest_memfd_private);
             }
-            block->guest_memfd_private = kvm_create_guest_memfd(
-                block->max_length, 0, errp);
+            block->guest_memfd_private =
+                kvm_create_guest_memfd_private(block->max_length, errp);
             if (block->guest_memfd_private < 0) {
                 qemu_mutex_unlock_ramlist();
                 return -1;
-- 
2.43.0