ptp_kvm implementation depends on hypercall using SMCCC. So we introduce a new SMCCC service ID. This doc explain how we define and use this new ID. Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx> --- Documentation/virt/kvm/arm/ptp_kvm.rst | 72 ++++++++++++++++++++++++++ 1 file changed, 72 insertions(+) create mode 100644 Documentation/virt/kvm/arm/ptp_kvm.rst diff --git a/Documentation/virt/kvm/arm/ptp_kvm.rst b/Documentation/virt/kvm/arm/ptp_kvm.rst new file mode 100644 index 000000000000..455591e2587a --- /dev/null +++ b/Documentation/virt/kvm/arm/ptp_kvm.rst @@ -0,0 +1,72 @@ +.. SPDX-License-Identifier: GPL-2.0 + +PTP_KVM support for arm64 +========================= + +PTP_KVM is used for time sync between guest and host in a high precison. +It needs get wall time and counter value from host and transfer these data +to guest via hypercall service. So one more hypercall service should be +added. + +This new SMCCC hypercall will be defined as: + +* ARM_SMCCC_HYP_KVM_PTP_FUNC_ID: 0xC6000001 + +As we only support 64-bits ptp_kvm client, so we choose SMC64/HVC64 +calling convention. + +ARM_SMCCC_HYP_KVM_PTP_FUNC_ID: + + ============ ======== ========== + Function ID: (uint32) 0xC6000001 + Arguments: (uint32) ARM_PTP_PHY_COUNTER(1) or ARM_PTP_VIRT_COUNTER(0) + which indicate acquiring physical counter or + virtual counter respectively. + return value: (uint64) NOT_SUPPORTED (-1) or two values of wall clock + time and counter cycle. + ============ ======== ========== + +Why we need PTP_KVM? +Currently, we offen use ntp (sync time with remote network clock) to sync time +in VM. But the precision of ntp is subject to network delay so it's difficult +to sync time in a high precision. + +kvm virtual ptp clock (ptp_kvm) offers another way to sync time in VM, in which +the remote clock locates in the host instead of remote network clock. It +targets to sync time between guest and host in virtualization environment and +in this way, we can also keep the time of all the VMs running in the same host +in sync. In general, the delay of communication between host and guest is quiet +small, so ptp_kvm can offer time sync precision up to in order of nanosecond. +Please keep in mind that ptp_kvm just limits itself to be a channel which +transmit the remote clock from host to guest and leaves the time sync jobs to +an application, eg. chrony, in usersapce of VM. + +How PTP_KVM works on arm64? +After ptp_kvm initialized, there will be a new device node under /dev called +ptp%d. A guest userspace service, like chrony, can use this device to get host +walltime, sometimes also counter cycle, which depends on the service it calls. +Then this guest userspace service can use those data to do the time sync for +guest. +Here is a rough sketch to show how kvm ptp clock works. + +|----------------------------| |--------------------------| +| guest userspace | | host | +|ioctl -> /dev/ptp%d | | | +| ^ | | | | +|----------------------------| | | +| | | guest kernel | | | +| | V (get host walltime/counter cycle) | +| ptp_kvm -> hypercall - - - - - - - - - - ->hypercall service | +| <- - - - - - - - - - - - | +|----------------------------| |--------------------------| + +1. time sync service in guest userspace call ptp device through /dev/ptp%d. +2. ptp_kvm module in guest recive this request then invoke hypercall to + route into host kernel to request host walltime/counter cycle. +3. ptp_kvm hypercall service in host response to the request and send data + back. +4. ptp (not ptp_kvm, ptp_kvm implemented on it and not shown above) in guest + copy the data to userspace. + +This ptp_kvm implementation focuses itself to step 2 and 3 and step 2 works +in guest comparing step 3 works in host kernel. -- 2.17.1
