[RFC PATCH v4 10/14] KVM: guest_memfd: Add a guest_memfd() flag to initialize it as mappable

Fuad Tabba
2024-12-13T16:48:36Z

Not all use cases require guest_memfd() to be mappable by the
host when first created. Add a new flag,
GUEST_MEMFD_FLAG_INIT_MAPPABLE, which when set on
KVM_CREATE_GUEST_MEMFD initializes the memory as mappable by the
host. Otherwise, memory is private until shared by the guest with
the host.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
Documentation/virt/kvm/api.rst | 4 ++++
include/uapi/linux/kvm.h | 1 +
tools/testing/selftests/kvm/guest_memfd_test.c | 7 +++++--
virt/kvm/guest_memfd.c | 6 +++++-
4 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 454c2aaa155e..60b65d9b8077 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6380,6 +6380,10 @@ most one mapping per page, i.e. binding multiple memory regions to a single
guest_memfd range is not allowed (any number of memory regions can be bound to
a single guest_memfd file, but the bound ranges must not overlap).

+If the capability KVM_CAP_GUEST_MEMFD_MAPPABLE is supported, then the flags
+field supports GUEST_MEMFD_FLAG_INIT_MAPPABLE, which initializes the memory
+as mappable by the host.
+
See KVM_SET_USER_MEMORY_REGION2 for additional details.

4.143 KVM_PRE_FAULT_MEMORY
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 021f8ef9979b..b34aed04ffa5 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1566,6 +1566,7 @@ struct kvm_memory_attributes {
#define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)

#define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
+#define GUEST_MEMFD_FLAG_INIT_MAPPABLE (1UL << 0)

struct kvm_create_guest_memfd {
__u64 size;
diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index ce687f8d248f..04b4111b7190 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -123,7 +123,7 @@ static void test_invalid_punch_hole(int fd, size_t page_size, size_t total_size)
static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
{
size_t page_size = getpagesize();
- uint64_t flag;
+ uint64_t flag = BIT(0);
size_t size;
int fd;

@@ -134,7 +134,10 @@ static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
size);
}

- for (flag = BIT(0); flag; flag <<= 1) {
+ if (kvm_has_cap(KVM_CAP_GUEST_MEMFD_MAPPABLE))
+ flag = GUEST_MEMFD_FLAG_INIT_MAPPABLE << 1;
+
+ for (; flag; flag <<= 1) {
fd = __vm_create_guest_memfd(vm, page_size, flag);
TEST_ASSERT(fd == -1 && errno == EINVAL,
"guest_memfd() with flag '0x%lx' should fail with EINVAL",
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 3d3645924db9..f33a577295b3 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -939,7 +939,8 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
goto err_gmem;
}

- if (IS_ENABLED(CONFIG_KVM_GMEM_MAPPABLE)) {
+ if (IS_ENABLED(CONFIG_KVM_GMEM_MAPPABLE) &&
+ (flags & GUEST_MEMFD_FLAG_INIT_MAPPABLE)) {
err = gmem_set_mappable(file_inode(file), 0, size >> PAGE_SHIFT);
if (err) {
fput(file);
@@ -968,6 +969,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
u64 flags = args->flags;
u64 valid_flags = 0;

+ if (IS_ENABLED(CONFIG_KVM_GMEM_MAPPABLE))
+ valid_flags |= GUEST_MEMFD_FLAG_INIT_MAPPABLE;
+
if (flags & ~valid_flags)
return -EINVAL;

--
2.47.1.613.gc27f4b7a9f-goog