KVM: guest_memfd: Allow mmap() on guest_memfd for x86 VMs with private memory

Allow mmap() on guest_memfd instances for x86 VMs with private memory as
the need to track private vs. shared state in the guest_memfd instance is
only pertinent to INIT_SHARED.  Doing mmap() on private memory isn't
terrible useful (yet!), but it's now possible, and will be desirable when
guest_memfd gains support for other VMA-based syscalls, e.g. mbind() to
set NUMA policy.

Lift the restriction now, before MMAP support is officially released, so
that KVM doesn't need to add another capability to enumerate support for
mmap() on private memory.

Fixes: 3d3a04f ("KVM: Allow and advertise support for host mmap() on guest_memfd files")
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -13942,10 +13942,11 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
 /*
- * KVM doesn't yet support mmap() on guest_memfd for VMs with private memory
- * (the private vs. shared tracking needs to be moved into guest_memfd).
+ * KVM doesn't yet support initializing guest_memfd memory as shared for VMs
+ * with private memory (the private vs. shared tracking needs to be moved into
+ * guest_memfd).
  */
-bool kvm_arch_supports_gmem_mmap(struct kvm *kvm)
+bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
 {
 	return !kvm_arch_has_private_mem(kvm);
 }

include/linux/kvm_host.h

@@ -729,7 +729,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
 #endif
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
-bool kvm_arch_supports_gmem_mmap(struct kvm *kvm);
+bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm);
+
+static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
+{
+	u64 flags = GUEST_MEMFD_FLAG_MMAP;
+
+	if (!kvm || kvm_arch_supports_gmem_init_shared(kvm))
+		flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
+
+	return flags;
+}
 #endif
 
 #ifndef kvm_arch_has_readonly_mem

virt/kvm/guest_memfd.c

@@ -485,7 +485,7 @@ static const struct inode_operations kvm_gmem_iops = {
 	.setattr	= kvm_gmem_setattr,
 };
 
-bool __weak kvm_arch_supports_gmem_mmap(struct kvm *kvm)
+bool __weak kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
 {
 	return true;
 }
@@ -549,13 +549,8 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 {
 	loff_t size = args->size;
 	u64 flags = args->flags;
-	u64 valid_flags = 0;
 
-	if (kvm_arch_supports_gmem_mmap(kvm))
-		valid_flags |= GUEST_MEMFD_FLAG_MMAP |
-			       GUEST_MEMFD_FLAG_INIT_SHARED;
-
-	if (flags & ~valid_flags)
+	if (flags & ~kvm_gmem_get_supported_flags(kvm))
 		return -EINVAL;
 
 	if (size <= 0 || !PAGE_ALIGNED(size))

virt/kvm/kvm_main.c

@@ -4929,11 +4929,7 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 	case KVM_CAP_GUEST_MEMFD:
 		return 1;
 	case KVM_CAP_GUEST_MEMFD_FLAGS:
-		if (!kvm || kvm_arch_supports_gmem_mmap(kvm))
-			return GUEST_MEMFD_FLAG_MMAP |
-			       GUEST_MEMFD_FLAG_INIT_SHARED;
-
-		return 0;
+		return kvm_gmem_get_supported_flags(kvm);
 #endif
 	default:
 		break;