[PATCH 1/9] KVM: arm64: Document PV-time interface

Steven Price
2019-08-02T14:51:34Z

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@arm.com>
---
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