[PATCH v4 04/10] KVM: guest_memfd: Add KVM capability to check if guest_memfd is shared

[Fuad Tabba <tabba@google.com>]

Add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which indicates
that the VM supports shared memory in guest_memfd, or that the
host can create VMs that support shared memory. Supporting shared
memory implies that memory can be mapped when shared with the
host.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
 include/uapi/linux/kvm.h | 1 +
 virt/kvm/kvm_main.c      | 4 ++++
 2 files changed, 5 insertions(+)

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 45e6d8fca9b9..117937a895da 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -929,6 +929,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_PRE_FAULT_MEMORY 236
 #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
 #define KVM_CAP_X86_GUEST_MODE 238
+#define KVM_CAP_GMEM_SHARED_MEM 239
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ba0327e2d0d3..38f0f402ea46 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4830,6 +4830,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 #ifdef CONFIG_KVM_PRIVATE_MEM
 	case KVM_CAP_GUEST_MEMFD:
 		return !kvm || kvm_arch_has_private_mem(kvm);
+#endif
+#ifdef CONFIG_KVM_GMEM_SHARED_MEM
+	case KVM_CAP_GMEM_SHARED_MEM:
+		return !kvm || kvm_arch_gmem_supports_shared_mem(kvm);
 #endif
 	default:
 		break;
-- 
2.48.1.601.g30ceb7b040-goog