[PATCH v6 7/7] KVM: guest_memfd: Add a guest_memfd() flag to initialize it as shared

[Fuad Tabba <tabba@xxxxxxxxxx>]

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

Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
---
 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                         | 12 ++++++++++++
 4 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 2b52eb77e29c..a5496d7d323b 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6386,6 +6386,10 @@ 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_GMEM_SHARED_MEM is supported, then the flags field
+supports GUEST_MEMFD_FLAG_INIT_SHARED, which initializes the memory as shared
+with the host, and thereby, mappable by it.
+
 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 117937a895da..22d7e33bf09c 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_SHARED		(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 38c501e49e0e..4a7fcd6aa372 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -159,7 +159,7 @@ 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;
 
@@ -170,7 +170,10 @@ 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_GMEM_SHARED_MEM))
+		flag = GUEST_MEMFD_FLAG_INIT_SHARED << 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 0487a08615f0..d7313e11c2cb 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -1045,6 +1045,15 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 		goto err_gmem;
 	}
 
+	if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM) &&
+	    (flags & GUEST_MEMFD_FLAG_INIT_SHARED)) {
+		err = kvm_gmem_offset_range_set_shared(file_inode(file), 0, size >> PAGE_SHIFT);
+		if (err) {
+			fput(file);
+			goto err_gmem;
+		}
+	}
+
 	kvm_get_kvm(kvm);
 	gmem->kvm = kvm;
 	xa_init(&gmem->bindings);
@@ -1066,6 +1075,9 @@ 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_SHARED_MEM))
+		valid_flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
+
 	if (flags & ~valid_flags)
 		return -EINVAL;
 
-- 
2.49.0.rc1.451.g8f38331e32-goog