[PATCH RFC 2/2] KVM: s390: Introduce KVM_VM_S390_LATE_MMAP

[David Hildenbrand]

Let's introduce a new VM type that allows user space to run KVM guests
without having to enable vm.alloc_pgste for all user space processes.
As long as user space follows these simple rules with the new VM type,
everything should be fine:
- Use only mmap for memory to be used with user memory regions
- Do all such mmap calls after the VM was created.

If user space fails to obey these rules, KVM will report -EFAULT
whenever an address in that incompatible range is accessed.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 Documentation/virtual/kvm/api.txt | 13 +++++++++++++
 arch/s390/kvm/kvm-s390.c          |  8 ++++++--
 include/uapi/linux/kvm.h          |  2 ++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 912b7df..056f391 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -117,6 +117,16 @@ In order to create user controlled virtual machines on S390, check
 KVM_CAP_S390_UCONTROL and use the flag KVM_VM_S390_UCONTROL as
 privileged user (CAP_SYS_ADMIN).
 
+When creating a virtual machine with the flag KVM_VM_S390_LATE_MMAP
+on s390, only memory mmap'ed after VM creation is guaranteed to be usable
+in user memory regions, because special page tables that might be
+necessary for virtualization will only be created for memory mmaped
+after this call. This avoids having to enable system wide vm.alloc_pgste
+in order to create KVM guests. Whether this type of VM can be created is
+indicated by the capability "KVM_CAP_S390_LATE_MMAP". If memory used for
+user memory regions is mmap'ed before VM creation or !mmap'ed memory is used
+for user memory regions, don't use this VM type.
+
 To use hardware assisted virtualization on MIPS (VZ ASE) rather than
 the default trap & emulate implementation (which changes the virtual
 memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the
@@ -985,6 +995,9 @@ the memory region are automatically reflected into the guest.  For example, an
 mmap() that affects the region will be made visible immediately.  Another
 example is madvise(MADV_DROP).
 
+See KVM_CREATE_VM for special handling related to KVM_VM_S390_LATE_MMAP
+on s390x.
+
 It is recommended to use this API instead of the KVM_SET_MEMORY_REGION ioctl.
 The KVM_SET_MEMORY_REGION does not allow fine grained control over memory
 allocation and is deprecated.
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 89684bb..75df5cd 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -389,6 +389,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_S390_USER_INSTR0:
 	case KVM_CAP_S390_CMMA_MIGRATION:
 	case KVM_CAP_S390_AIS:
+	case KVM_CAP_S390_LATE_MMAP:
 		r = 1;
 		break;
 	case KVM_CAP_S390_MEM_OP:
@@ -1796,6 +1797,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	int i, rc;
 	char debug_name[16];
 	static unsigned long sca_offset;
+	bool mixed_pgtables = false;
 
 	rc = -EINVAL;
 #ifdef CONFIG_KVM_S390_UCONTROL
@@ -1804,11 +1806,13 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	if ((type & KVM_VM_S390_UCONTROL) && (!capable(CAP_SYS_ADMIN)))
 		goto out_err;
 #else
-	if (type)
+	if (type & KVM_VM_S390_LATE_MMAP)
+		mixed_pgtables = true;
+	else if (type)
 		goto out_err;
 #endif
 
-	rc = s390_enable_sie(false);
+	rc = s390_enable_sie(mixed_pgtables);
 	if (rc)
 		goto out_err;
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 2b8dc1c..436ba30 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -726,6 +726,7 @@ struct kvm_ppc_resize_hpt {
 
 /* machine type bits, to be used as argument to KVM_CREATE_VM */
 #define KVM_VM_S390_UCONTROL	1
+#define KVM_VM_S390_LATE_MMAP	2
 
 /* on ppc, 0 indicate default, 1 should force HV and 2 PR */
 #define KVM_VM_PPC_HV 1
@@ -925,6 +926,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_X86_GUEST_MWAIT 143
 #define KVM_CAP_ARM_USER_IRQ 144
 #define KVM_CAP_S390_CMMA_MIGRATION 145
+#define KVM_CAP_S390_LATE_MMAP 200
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.9.3