KVM: s390: fake memslot for ucontrol VMs

Create a fake memslot for ucontrol VMs. The fake memslot identity-maps
userspace.

Now memslots will always be present, and ucontrol is not a special case
anymore.

Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20250123144627.312456-4-imbrenda@linux.ibm.com
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20250123144627.312456-4-imbrenda@linux.ibm.com>

Author: Claudio Imbrenda

Documentation/virt/kvm/api.rst

@@ -1419,7 +1419,7 @@ fetch) is injected in the guest.
 S390:
 ^^^^^
 
-Returns -EINVAL if the VM has the KVM_VM_S390_UCONTROL flag set.
+Returns -EINVAL or -EEXIST if the VM has the KVM_VM_S390_UCONTROL flag set.
 Returns -EINVAL if called on a protected VM.
 
 4.36 KVM_SET_TSS_ADDR

arch/s390/include/asm/kvm_host.h

@@ -30,6 +30,8 @@
 #define KVM_S390_ESCA_CPU_SLOTS 248
 #define KVM_MAX_VCPUS 255
 
+#define KVM_INTERNAL_MEM_SLOTS 1
+
 /*
  * These seem to be used for allocating ->chip in the routing table, which we
  * don't use. 1 is as small as we can get to reduce the needed memory. If we

arch/s390/kvm/kvm-s390.c

@@ -3428,8 +3428,20 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	VM_EVENT(kvm, 3, "vm created with type %lu", type);
 
 	if (type & KVM_VM_S390_UCONTROL) {
+		struct kvm_userspace_memory_region2 fake_memslot = {
+			.slot = KVM_S390_UCONTROL_MEMSLOT,
+			.guest_phys_addr = 0,
+			.userspace_addr = 0,
+			.memory_size = ALIGN_DOWN(TASK_SIZE, _SEGMENT_SIZE),
+			.flags = 0,
+		};
+
 		kvm->arch.gmap = NULL;
 		kvm->arch.mem_limit = KVM_S390_NO_MEM_LIMIT;
+		/* one flat fake memslot covering the whole address-space */
+		mutex_lock(&kvm->slots_lock);
+		KVM_BUG_ON(kvm_set_internal_memslot(kvm, &fake_memslot), kvm);
+		mutex_unlock(&kvm->slots_lock);
 	} else {
 		if (sclp.hamax == U64_MAX)
 			kvm->arch.mem_limit = TASK_SIZE_MAX;
@@ -5854,7 +5866,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 {
 	gpa_t size;
 
-	if (kvm_is_ucontrol(kvm))
+	if (kvm_is_ucontrol(kvm) && new->id < KVM_USER_MEM_SLOTS)
 		return -EINVAL;
 
 	/* When we are protected, we should not change the memory slots */
@@ -5906,6 +5918,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 {
 	int rc = 0;
 
+	if (kvm_is_ucontrol(kvm))
+		return;
+
 	switch (change) {
 	case KVM_MR_DELETE:
 		rc = gmap_unmap_segment(kvm->arch.gmap, old->base_gfn * PAGE_SIZE,

arch/s390/kvm/kvm-s390.h

@@ -20,6 +20,8 @@
 #include <asm/processor.h>
 #include <asm/sclp.h>
 
+#define KVM_S390_UCONTROL_MEMSLOT (KVM_USER_MEM_SLOTS + 0)
+
 static inline void kvm_s390_fpu_store(struct kvm_run *run)
 {
 	fpu_stfpc(&run->s.regs.fpc);