[PATCH v14 19/21] KVM: Introduce the KVM capability KVM_CAP_GMEM_MMAP

[Fuad Tabba <tabba@google.com>]

Introduce the new KVM capability KVM_CAP_GMEM_MMAP. This capability
signals to userspace that a KVM instance supports host userspace mapping
of guest_memfd-backed memory.

The availability of this capability is determined per architecture, and
its enablement for a specific guest_memfd instance is controlled by the
GUEST_MEMFD_FLAG_MMAP flag at creation time.

Update the KVM API documentation to detail the KVM_CAP_GMEM_MMAP
capability, the associated GUEST_MEMFD_FLAG_MMAP, and provide essential
information regarding support for mmap in guest_memfd.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 Documentation/virt/kvm/api.rst | 9 +++++++++
 include/uapi/linux/kvm.h       | 1 +
 virt/kvm/kvm_main.c            | 4 ++++
 3 files changed, 14 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 43ed57e048a8..5169066b53b2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6407,6 +6407,15 @@ 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).
 
+When the capability KVM_CAP_GMEM_MMAP is supported, the 'flags' field supports
+GUEST_MEMFD_FLAG_MMAP.  Setting this flag on guest_memfd creation enables mmap()
+and faulting of guest_memfd memory to host userspace.
+
+When the KVM MMU performs a PFN lookup to service a guest fault and the backing
+guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be
+consumed from guest_memfd, regardless of whether it is a shared or a private
+fault.
+
 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 3beafbf306af..698dd407980f 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -960,6 +960,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_ARM_EL2 240
 #define KVM_CAP_ARM_EL2_E2H0 241
 #define KVM_CAP_RISCV_MP_STATE_RESET 242
+#define KVM_CAP_GMEM_MMAP 243
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 46bddac1dacd..f1ac872e01e9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4916,6 +4916,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 #ifdef CONFIG_KVM_GMEM
 	case KVM_CAP_GUEST_MEMFD:
 		return !kvm || kvm_arch_supports_gmem(kvm);
+#endif
+#ifdef CONFIG_KVM_GMEM_SUPPORTS_MMAP
+	case KVM_CAP_GMEM_MMAP:
+		return !kvm || kvm_arch_supports_gmem_mmap(kvm);
 #endif
 	default:
 		break;
-- 
2.50.0.727.gbf7dc18ff4-goog