KVM: s390: API documentation

This patch adds Documentation/s390/kvm.txt, which describes specifics of kvm's
user interface that are unique to s390 architecture.
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

Documentation/s390/kvm.txt

+*** BIG FAT WARNING ***
+The kvm module is currently in EXPERIMENTAL state for s390. This means that
+the interface to the module is not yet considered to remain stable. Thus, be
+prepared that we keep breaking your userspace application and guest
+compatibility over and over again until we feel happy with the result. Make sure
+your guest kernel, your host kernel, and your userspace launcher are in a
+consistent state.
+
+This Documentation describes the unique ioctl calls to /dev/kvm, the resulting
+kvm-vm file descriptors, and the kvm-vcpu file descriptors that differ from x86.
+
+1. ioctl calls to /dev/kvm
+KVM does support the following ioctls on s390 that are common with other
+architectures and do behave the same:
+KVM_GET_API_VERSION
+KVM_CREATE_VM		(*) see note
+KVM_CHECK_EXTENSION
+KVM_GET_VCPU_MMAP_SIZE
+
+Notes:
+* KVM_CREATE_VM may fail on s390, if the calling process has multiple
+threads and has not called KVM_S390_ENABLE_SIE before.
+
+In addition, on s390 the following architecture specific ioctls are supported:
+ioctl:		KVM_S390_ENABLE_SIE
+args:		none
+see also:	include/linux/kvm.h
+This call causes the kernel to switch on PGSTE in the user page table. This
+operation is needed in order to run a virtual machine, and it requires the
+calling process to be single-threaded. Note that the first call to KVM_CREATE_VM
+will implicitly try to switch on PGSTE if the user process has not called
+KVM_S390_ENABLE_SIE before. User processes that want to launch multiple threads
+before creating a virtual machine have to call KVM_S390_ENABLE_SIE, or will
+observe an error calling KVM_CREATE_VM. Switching on PGSTE is a one-time
+operation, is not reversible, and will persist over the entire lifetime of
+the calling process. It does not have any user-visible effect other than a small
+performance penalty.
+
+2. ioctl calls to the kvm-vm file descriptor
+KVM does support the following ioctls on s390 that are common with other
+architectures and do behave the same:
+KVM_CREATE_VCPU
+KVM_SET_USER_MEMORY_REGION      (*) see note
+KVM_GET_DIRTY_LOG		(**) see note
+
+Notes:
+*  kvm does only allow exactly one memory slot on s390, which has to start
+   at guest absolute address zero and at a user address that is aligned on any
+   page boundary. This hardware "limitation" allows us to have a few unique
+   optimizations. The memory slot doesn't have to be filled
+   with memory actually, it may contain sparse holes. That said, with different
+   user memory layout this does still allow a large flexibility when
+   doing the guest memory setup.
+** KVM_GET_DIRTY_LOG doesn't work properly yet. The user will receive an empty
+log. This ioctl call is only needed for guest migration, and we intend to
+implement this one in the future.
+
+In addition, on s390 the following architecture specific ioctls for the kvm-vm
+file descriptor are supported:
+ioctl:		KVM_S390_INTERRUPT
+args:		struct kvm_s390_interrupt *
+see also:	include/linux/kvm.h
+This ioctl is used to submit a floating interrupt for a virtual machine.
+Floating interrupts may be delivered to any virtual cpu in the configuration.
+Only some interrupt types defined in include/linux/kvm.h make sense when
+submitted as floating interrupts. The following interrupts are not considered
+to be useful as floating interrupts, and a call to inject them will result in
+-EINVAL error code: program interrupts and interprocessor signals. Valid
+floating interrupts are:
+KVM_S390_INT_VIRTIO
+KVM_S390_INT_SERVICE
+
+3. ioctl calls to the kvm-vcpu file descriptor
+KVM does support the following ioctls on s390 that are common with other
+architectures and do behave the same:
+KVM_RUN
+KVM_GET_REGS
+KVM_SET_REGS
+KVM_GET_SREGS
+KVM_SET_SREGS
+KVM_GET_FPU
+KVM_SET_FPU
+
+In addition, on s390 the following architecture specific ioctls for the
+kvm-vcpu file descriptor are supported:
+ioctl:		KVM_S390_INTERRUPT
+args:		struct kvm_s390_interrupt *
+see also:	include/linux/kvm.h
+This ioctl is used to submit an interrupt for a specific virtual cpu.
+Only some interrupt types defined in include/linux/kvm.h make sense when
+submitted for a specific cpu. The following interrupts are not considered
+to be useful, and a call to inject them will result in -EINVAL error code:
+service processor calls and virtio interrupts. Valid interrupt types are:
+KVM_S390_PROGRAM_INT
+KVM_S390_SIGP_STOP
+KVM_S390_RESTART
+KVM_S390_SIGP_SET_PREFIX
+KVM_S390_INT_EMERGENCY
+
+ioctl:		KVM_S390_STORE_STATUS
+args:		unsigned long
+see also:	include/linux/kvm.h
+This ioctl stores the state of the cpu at the guest real address given as
+argument, unless one of the following values defined in include/linux/kvm.h
+is given as arguement:
+KVM_S390_STORE_STATUS_NOADDR - the CPU stores its status to the save area in
+absolute lowcore as defined by the principles of operation
+KVM_S390_STORE_STATUS_PREFIXED - the CPU stores its status to the save area in
+its prefix page just like the dump tool that comes with zipl. This is useful
+to create a system dump for use with lkcdutils or crash.
+
+ioctl:		KVM_S390_SET_INITIAL_PSW
+args:		struct kvm_s390_psw *
+see also:	include/linux/kvm.h
+This ioctl can be used to set the processor status word (psw) of a stopped cpu
+prior to running it with KVM_RUN. Note that this call is not required to modify
+the psw during sie intercepts that fall back to userspace because struct kvm_run
+does contain the psw, and this value is evaluated during reentry of KVM_RUN
+after the intercept exit was recognized.
+
+ioctl:		KVM_S390_INITIAL_RESET
+args:		none
+see also:	include/linux/kvm.h
+This ioctl can be used to perform an initial cpu reset as defined by the
+principles of operation. The target cpu has to be in stopped state.