Add CAP to enumerate supported SET_MEMORY_ATTRIBUTES2 flags, so userspace
can find out which flags are supported when sending the
KVM_SET_MEMORY_ATTRIBUTES2 ioctl to a guest_memfd.

These flags are only supported by guest_memfd, hence, if
vm_memory_attributes is enabled, return 0 - no flags are supported when
KVM_SET_MEMORY_ATTRIBUTES2 is sent to a VM fd.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 Documentation/virt/kvm/api.rst |  3 +++
 include/linux/kvm_host.h       | 12 ++++++++++++
 include/uapi/linux/kvm.h       |  1 +
 virt/kvm/kvm_main.c            |  5 +++++
 4 files changed, 21 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 90587a9c09d3f..73101d6e649b7 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6680,6 +6680,9 @@ guarantees are made on offset ranges that do not have memory allocated
 and range [0x0000, 0x3000) was set to shared, the content mode would
 apply to only to offset ranges [0x0000, 0x1000) and [0x2000, 0x3000).
 
+The supported content modes can be queried using
+``KVM_CAP_MEMORY_ATTRIBUTES2_FLAGS``.
+
 See also: :ref: `KVM_SET_MEMORY_ATTRIBUTES`.
 
 .. _kvm_run:
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4ba3c4b303f4a..d671c9795dfce 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2536,6 +2536,18 @@ static inline u64 kvm_supported_mem_attributes(struct kvm *kvm)
 	return 0;
 }
 
+static inline u64 kvm_supported_set_mem_attributes2_flags(struct kvm *kvm)
+{
+	if (!IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
+		return 0;
+
+	if (!kvm)
+		return KVM_SET_MEMORY_ATTRIBUTES2_ZERO |
+		       KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE;
+
+	return kvm_arch_gmem_supported_content_modes(kvm);
+}
+
 typedef unsigned long (kvm_get_memory_attributes_t)(struct kvm *kvm, gfn_t gfn);
 DECLARE_STATIC_CALL(__kvm_get_memory_attributes, kvm_get_memory_attributes_t);
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 0fc9ad4ea0d93..4d1e09d022c48 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -991,6 +991,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_S390_KEYOP 247
 #define KVM_CAP_MEMORY_ATTRIBUTES2 248
 #define KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES 249
+#define KVM_CAP_MEMORY_ATTRIBUTES2_FLAGS 250
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 85c14197587d4..6607127eaaeed 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4980,6 +4980,11 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 			return 0;
 
 		return kvm_supported_mem_attributes(kvm);
+	case KVM_CAP_MEMORY_ATTRIBUTES2_FLAGS:
+		if (vm_memory_attributes)
+			return 0;
+
+		return kvm_supported_set_mem_attributes2_flags(kvm);
 #endif
 	default:
 		break;

-- 
2.53.0.1018.g2bb0e51243-goog