[PATCH v14 09/10] doc: add ptp_kvm introduction for arm64 support

[Jianyong Wu]

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