[RFC PATCH v3 3/5] KVM: guest_memfd: Advertise support for KVM_LINK_GUEST_MEMFD

[Tao Gong]

From: Vishal Annapurve <vannapurve@xxxxxxxxxx>

Advertise KVM_CAP_GUEST_MEMFD_LINK via KVM_CHECK_EXTENSION so that
userspace VMMs and selftests can discover support for linking
guest_memfd instances across VMs via KVM_LINK_GUEST_MEMFD.

Signed-off-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>
Co-developed-by: Tao Gong <gongtao@xxxxxxxxxx>
Signed-off-by: Tao Gong <gongtao@xxxxxxxxxx>
---
 Documentation/virt/kvm/api.rst | 8 ++++++++
 include/uapi/linux/kvm.h       | 1 +
 virt/kvm/kvm_main.c            | 2 ++
 3 files changed, 11 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 7c527a1e7c487..bc54212e48c32 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -9738,6 +9738,14 @@ take care to differentiate between these cases.
 The presence of this capability indicates that the nested KVM guest can
 start in ESA mode.
 
+8.48 KVM_CAP_GUEST_MEMFD_LINK
+-----------------------------
+
+:Architectures: all
+
+The presence of this capability indicates that KVM supports linking an
+existing guest_memfd file to another VM via KVM_LINK_GUEST_MEMFD.
+
 9. Known KVM API problems
 =========================
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 53fe8dce57db2..09926fa85fe6c 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1000,6 +1000,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_PPC_COMPAT_CAPS 250
 #define KVM_CAP_ARM_PMU_V3_STRICT 251
 #define KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES 252
+#define KVM_CAP_GUEST_MEMFD_LINK 253
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70521371b64ab..135e7bb7609a8 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5008,6 +5008,8 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 			return 0;
 
 		return KVM_MEMORY_ATTRIBUTE_PRIVATE;
+	case KVM_CAP_GUEST_MEMFD_LINK:
+		return 1;
 #endif
 	default:
 		break;
-- 
2.56.0.257.gf7497c7d27-goog