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diff --git a/Documentation/s390/kvm.txt b/Documentation/s390/kvm.txt new file mode 100644 index 000000000000..6f5ceb0f09fc --- /dev/null +++ b/Documentation/s390/kvm.txt @@ -0,0 +1,125 @@ +*** 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. |