Introduce a paravirtualization interface for KVM/arm64 based on the "Arm Paravirtualized Time for Arm-Base Systems" specification DEN 0057A. This only adds the details about "Stolen Time" as the details of "Live Physical Time" have not been fully agreed. User space can specify a reserved area of memory for the guest and inform KVM to populate the memory with information on time that the host kernel has stolen from the guest. A hypercall interface is provided for the guest to interrogate the hypervisor's support for this interface and the location of the shared memory structures. Signed-off-by: Steven Price <steven.price@xxxxxxx> --- Documentation/virtual/kvm/arm/pvtime.txt | 107 +++++++++++++++++++++++ 1 file changed, 107 insertions(+) create mode 100644 Documentation/virtual/kvm/arm/pvtime.txt diff --git a/Documentation/virtual/kvm/arm/pvtime.txt b/Documentation/virtual/kvm/arm/pvtime.txt new file mode 100644 index 000000000000..e6ae9799e1d5 --- /dev/null +++ b/Documentation/virtual/kvm/arm/pvtime.txt @@ -0,0 +1,107 @@ +Paravirtualized time support for arm64 +====================================== + +Arm specification DEN0057/A defined a standard for paravirtualised time +support for Aarch64 guests: + +https://developer.arm.com/docs/den0057/a + +KVM/Arm64 implements the stolen time part of this specification by providing +some hypervisor service calls to support a paravirtualized guest obtaining a +view of the amount of time stolen from its execution. + +Two new SMCCC compatible hypercalls are defined: + +PV_FEATURES 0xC5000020 +PV_TIME_ST 0xC5000022 + +These are only available in the SMC64/HVC64 calling convention as +paravirtualized time is not available to 32 bit Arm guests. + +PV_FEATURES + Function ID: (uint32) : 0xC5000020 + PV_func_id: (uint32) : Either PV_TIME_LPT or PV_TIME_ST + Return value: (int32) : NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant + PV-time feature is supported by the hypervisor. + +PV_TIME_ST + Function ID: (uint32) : 0xC5000022 + Return value: (int64) : IPA of the stolen time data structure for this + (V)CPU. On failure: + NOT_SUPPORTED (-1) + +Stolen Time +----------- + +The structure pointed to by the PV_TIME_ST hypercall is as follows: + + Field | Byte Length | Byte Offset | Description + ----------- | ----------- | ----------- | -------------------------- + Revision | 4 | 0 | Must be 0 for version 0.1 + Attributes | 4 | 4 | Must be 0 + Stolen time | 8 | 8 | Stolen time in unsigned + | | | nanoseconds indicating how + | | | much time this VCPU thread + | | | was involuntarily not + | | | running on a physical CPU. + +The structure will be updated by the hypervisor periodically as time is stolen +from the VCPU. It will be present within a reserved region of the normal +memory given to the guest. The guest should not attempt to write into this +memory. There is a structure by VCPU of the guest. + +User space interface +==================== + +User space can request that KVM provide the paravirtualized time interface to +a guest by creating a KVM_DEV_TYPE_ARM_PV_TIME device, for example: + + struct kvm_create_device pvtime_device = { + .type = KVM_DEV_TYPE_ARM_PV_TIME, + .attr = 0, + .flags = 0, + }; + + pvtime_fd = ioctl(vm_fd, KVM_CREATE_DEVICE, &pvtime_device); + +The guest IPA of the structures must be given to KVM. This is the base address +of an array of stolen time structures (one for each VCPU). For example: + + struct kvm_device_attr st_base = { + .group = KVM_DEV_ARM_PV_TIME_PADDR, + .attr = KVM_DEV_ARM_PV_TIME_ST, + .addr = (u64)(unsigned long)&st_paddr + }; + + ioctl(pvtime_fd, KVM_SET_DEVICE_ATTR, &st_base); + +For migration (or save/restore) of a guest it is necessary to save the contents +of the shared page(s) and later restore them. KVM_DEV_ARM_PV_TIME_STATE_SIZE +provides the size of this data and KVM_DEV_ARM_PV_TIME_STATE allows the state +to be read/written. + +It is also necessary for the physical address to be set identically when +restoring. + + void *save_state(int fd, u64 attr, u32 *size) { + struct kvm_device_attr get_size = { + .group = KVM_DEV_ARM_PV_TIME_STATE_SIZE, + .attr = attr, + .addr = (u64)(unsigned long)size + }; + + ioctl(fd, KVM_GET_DEVICE_ATTR, get_size); + + void *buffer = malloc(*size); + + struct kvm_device_attr get_state = { + .group = KVM_DEV_ARM_PV_TIME_STATE, + .attr = attr, + .addr = (u64)(unsigned long)size + }; + + ioctl(fd, KVM_GET_DEVICE_ATTR, buffer); + } + + void *st_state = save_state(pvtime_fd, KVM_DEV_ARM_PV_TIME_ST, &st_size); + -- 2.20.1
