[PATCH RFC 01/12] accel/kvm: Decouple guest_memfd checks from memory attribute checks

[Michael Roth <michael.roth@amd.com>]

Currently QEMU supports using guest_memfd internally (separately from
user-specified memory backends) to handle private memory for
confidential VMs, and as a result has checks for guest_memfd support
merged with checks to see if KVM can handle mapping private memory (as
determined by KVM_MEMORY_ATTRIBUTE_PRIVATE).

Future QEMU support will allow using guest_memfd not just for private
memory, but as mmap()'able memory that can be used by non-confidential
guests as well.

In prep for this, split the checks for guest_memfd out from the check
for KVM_MEMORY_ATTRIBUTE_PRIVATE, and rename the current
kvm_create_guest_memfd() to kvm_create_guest_memfd_private() to
self-document current behavior/expectations and disambiguate from future
helpers intended for creating a guest_memfd to handle non-private/shared
memory. While there, fix up the missing error_setg() handling in the
stub functions.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 accel/kvm/kvm-all.c     | 20 +++++++++++++++++---
 accel/stubs/kvm-stub.c  |  3 ++-
 include/system/kvm.h    |  2 +-
 include/system/memory.h |  5 +++--
 system/physmem.c        |  8 ++++----
 5 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 585f1cea35..02911ff6e3 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -795,6 +795,11 @@ static int kvm_mem_flags(MemoryRegion *mr)
     }
     if (memory_region_has_guest_memfd(mr)) {
         assert(kvm_guest_memfd_supported);
+        /*
+         * memory_region_has_guest_memfd() 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;
@@ -3066,8 +3071,7 @@ static int kvm_init(AccelState *as, MachineState *ms)
     kvm_supported_memory_attributes = kvm_vm_check_extension(s, KVM_CAP_MEMORY_ATTRIBUTES);
     kvm_guest_memfd_supported =
         kvm_vm_check_extension(s, KVM_CAP_GUEST_MEMFD) &&
-        kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2) &&
-        (kvm_supported_memory_attributes & KVM_MEMORY_ATTRIBUTE_PRIVATE);
+        kvm_vm_check_extension(s, KVM_CAP_USER_MEMORY2);
     kvm_pre_fault_memory_supported = kvm_vm_check_extension(s, KVM_CAP_PRE_FAULT_MEMORY);
 
     if (s->kernel_irqchip_split == ON_OFF_AUTO_AUTO) {
@@ -4854,7 +4858,7 @@ void kvm_mark_guest_state_protected(void)
     kvm_state->guest_state_protected = true;
 }
 
-int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
+static int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
 {
     int fd;
     struct kvm_create_guest_memfd guest_memfd = {
@@ -4875,3 +4879,13 @@ 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 c4617caac6..1940bcbd2c 100644
--- a/accel/stubs/kvm-stub.c
+++ b/accel/stubs/kvm-stub.c
@@ -139,7 +139,8 @@ bool kvm_hwpoisoned_mem(void)
     return false;
 }
 
-int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp)
+int kvm_create_guest_memfd_private(uint64_t size, Error **errp)
 {
+    error_setg(errp, "guest_memfd is not supported for this configuration");
     return -ENOSYS;
 }
diff --git a/include/system/kvm.h b/include/system/kvm.h
index 5fa33eddda..aeb0c7ca8f 100644
--- a/include/system/kvm.h
+++ b/include/system/kvm.h
@@ -561,7 +561,7 @@ 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/include/system/memory.h b/include/system/memory.h
index 1417132f6d..24c68720aa 100644
--- a/include/system/memory.h
+++ b/include/system/memory.h
@@ -1745,9 +1745,10 @@ bool memory_region_is_protected(const MemoryRegion *mr);
 
 /**
  * memory_region_has_guest_memfd: check whether a memory region has guest_memfd
- *     associated
+ *     associated with it for handling private memory
  *
- * Returns %true if a memory region's ram_block has valid guest_memfd assigned.
+ * Returns %true if a memory region's ram_block has valid guest_memfd assigned
+ * for handling private memory.
  *
  * @mr: the memory region being queried
  */
diff --git a/system/physmem.c b/system/physmem.c
index 7bcbf87573..04c7c38721 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -2202,8 +2202,8 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
             goto out_free;
         }
 
-        new_block->guest_memfd = kvm_create_guest_memfd(new_block->max_length,
-                                                        0, errp);
+        new_block->guest_memfd = kvm_create_guest_memfd_private(new_block->max_length,
+                                                                errp);
         if (new_block->guest_memfd < 0) {
             qemu_mutex_unlock_ramlist();
             goto out_free;
@@ -2835,8 +2835,8 @@ int ram_block_rebind(Error **errp)
             if (block->guest_memfd >= 0) {
                 close(block->guest_memfd);
             }
-            block->guest_memfd = kvm_create_guest_memfd(block->max_length,
-                                                        0, errp);
+            block->guest_memfd = kvm_create_guest_memfd_private(block->max_length,
+                                                                errp);
             if (block->guest_memfd < 0) {
                 qemu_mutex_unlock_ramlist();
                 return -1;
-- 
2.43.0