doc: add enable PTM tutorial

Restore PTM tutorial removed for the 2.5 release (it's tested as
working now)

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/develop.rst

@@ -77,6 +77,7 @@ Enable ACRN Features
    tutorials/acrn-secure-boot-with-efi-stub
    tutorials/pre-launched-rt
    tutorials/enable_ivshmem
+   tutorials/enable_ptm
 
 Debug
 *****

doc/tutorials/enable_ptm.rst

@@ -0,0 +1,87 @@
+.. _enable-ptm:
+
+Enable PCIe Precision Time Management
+#####################################
+
+The PCI Express (PCIe) specification defines a Precision Time Measurement (PTM)
+mechanism that lets you coordinate and synchronize events across multiple PCI
+components within the same system with very fine time precision.
+
+ACRN adds PCIe root port emulation in the hypervisor to support the PTM feature
+and emulates a simple PTM hierarchy.  ACRN enables PTM in a Guest VM if the user
+sets the ``enable_ptm`` option when passing through a device to a post-launched
+VM and :ref:`vm.PTM` is enabled in the scenario configuration. When you enable
+PTM, the passthrough device is connected to a virtual root port instead of the host
+bridge as it normally would.
+
+Here is an example launch script that configures a supported Ethernet card for
+passthrough and enables PTM on it:
+
+.. code-block:: bash
+   :emphasize-lines: 9-11,17
+
+   declare -A passthru_vpid
+   declare -A passthru_bdf
+   passthru_vpid=(
+    ["ethptm"]="8086 15f2"
+    )
+   passthru_bdf=(
+    ["ethptm"]="0000:aa:00.0"
+    )
+   echo ${passthru_vpid["ethptm"]} > /sys/bus/pci/drivers/pci-stub/new_id
+   echo ${passthru_bdf["ethptm"]} > /sys/bus/pci/devices/${passthru_bdf["ethptm"]}/driver/unbind
+   echo ${passthru_bdf["ethptm"]} > /sys/bus/pci/drivers/pci-stub/bind
+
+   acrn-dm -A -m $mem_size -s 0:0,hostbridge \
+      -s 3,virtio-blk,uos-test.img \
+      -s 4,virtio-net,tap0 \
+      -s 5,virtio-console,@stdio:stdio_port \
+      -s 6,passthru,a9/00/0,enable_ptm \
+      --ovmf /usr/share/acrn/bios/OVMF.fd
+
+.. important:: By default, the :ref:`vm.PTM` option is disabled in ACRN VMs. Use the
+    :ref:`ACRN configuration tool <acrn_configuration_tool>` to enable PTM
+    in the scenario XML file that configures the Guest VM.
+
+Here is the bus hierarchy in the Guest VM (as shown by the ``lspci`` command)::
+
+   lspci -tv
+   -[0000:00]-+-00.0  Network Appliance Corporation Device 1275
+              +-03.0  Red Hat, Inc. Virtio block device
+              +-04.0  Red Hat, Inc. Virtio network device
+              +-05.0  Red Hat, Inc. Virtio console
+              \-06.0-[01]----00.0  Intel Corporation Device 15f2
+
+(Instead of ``Device 15f2`` you might see ``Ethernet Controller I225LM``.)
+
+You can also verify that PTM was enabled by using ``dmesg`` in the guest VM::
+
+    dmesg | grep -i ptm
+    [ 1.555284] pci_ptm_init: 00:00.00, ispcie=1, type=0x4
+    [ 1.555356] Cannot find PTM ext cap.
+    [ 1.561311] pci_ptm_init: 00:03.00, ispcie=0, type=0x0
+    [ 1.567146] pci_ptm_init: 00:04.00, ispcie=0, type=0x0
+    [ 1.572983] pci_ptm_init: 00:05.00, ispcie=0, type=0x0
+    [ 1.718038] pci_ptm_init: 00:06.00, ispcie=1, type=0x4
+    [ 1.722034] ptm is ptm_root.
+    [ 1.723033] Condition-2: ptm is enabled.
+    [ 1.723052] pci 0000:00:06.0: PTM enabled (root), 4ns granularity
+    [ 1.766438] pci_ptm_init: a9:00.00, ispcie=1, type=0x0
+    [ 5.715000] igc_probe enable ptm.
+    [ 5.715068] pci_enable_ptm: a9:00.00, ispcie=1, type=0x0
+    [ 5.715294] ptm is enabled on endpoint device.
+    [ 5.715371] igc 0000:a9:00.0: PTM enabled, 4ns granularity
+
+PTM Implementation Notes
+************************
+
+To simplify the implementation, the virtual root port only supports the most
+basic PCIe configuration and operation, in addition to PTM capabilities.
+
+To use PTM in a virtualized environment, you may want to first verify that PTM
+is supported by the device and is enabled on the bare metal machine and in the
+Guest VM kernel (e.g., ``CONFIG_PCIE_PTM=y`` option is set in the Linux kernel).
+
+You can find more details about the PTM implementation in the
+:ref:`ACRN HLD PCIe PTM documentation <PCIe PTM implementation>`.
+