[PATCH] Documentation: PCI: add vmd documentation

[Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>]

Adding documentation for the Intel VMD driver and updating the index
file to include it.

Signed-off-by: Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>
---
 Documentation/PCI/controller/vmd.rst | 51 ++++++++++++++++++++++++++++
 Documentation/PCI/index.rst          |  1 +
 2 files changed, 52 insertions(+)
 create mode 100644 Documentation/PCI/controller/vmd.rst

diff --git a/Documentation/PCI/controller/vmd.rst b/Documentation/PCI/controller/vmd.rst
new file mode 100644
index 000000000000..e1a019035245
--- /dev/null
+++ b/Documentation/PCI/controller/vmd.rst
@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+=================================================================
+Linux Base Driver for the Intel(R) Volume Management Device (VMD)
+=================================================================
+
+Intel vmd Linux driver.
+
+Contents
+========
+
+- Overview
+- Features
+- Limitations
+
+The Intel VMD provides the means to provide volume management across separate
+PCI Express HBAs and SSDs without requiring operating system support or
+communication between drivers. It does this by obscuring each storage
+controller from the OS, but allowing a single driver to be loaded that would
+control each storage controller. A Volume Management Device (VMD) provides a
+single device for a single storage driver. The VMD resides in the IIO root
+complex and it appears to the OS as a root bus integrated endpoint. In the IIO,
+the VMD is in a central location to manipulate access to storage devices which
+may be attached directly to the IIO or indirectly through the PCH. Instead of
+allowing individual storage devices to be detected by the OS and allow it to
+load a separate driver instance for each, the VMD provides configuration
+settings to allow specific devices and root ports on the root bus to be
+invisible to the OS.
+
+VMD works by creating separate PCI domains for each VMD device in the system.
+This makes VMD look more like a host bridge than an endpoint so VMD must try
+to adhere to the ACPI Operating System Capabilities (_OSC) flags of the system.
+A couple of the _OSC flags regard hotplug support.  Hotplug is a feature that
+is always enabled when using VMD regardless of the _OSC flags.
+
+Features
+========
+
+- Virtualization
+- MSIX interrupts
+- Power Management
+- Hotplug
+
+Limitations
+===========
+
+When VMD is enabled and used in a hypervisor the _OSC flags provided by the
+hypervisor BIOS may not be correct. The most critical of these flags are the
+hotplug bits. If these bits are incorrect then the storage devices behind the
+VMD will not be able to be hotplugged. The driver always supports hotplug for
+the devices behind it so the hotplug bits reported by the OS are not used.
diff --git a/Documentation/PCI/index.rst b/Documentation/PCI/index.rst
index e73f84aebde3..6558adc703f9 100644
--- a/Documentation/PCI/index.rst
+++ b/Documentation/PCI/index.rst
@@ -18,3 +18,4 @@ PCI Bus Subsystem
    pcieaer-howto
    endpoint/index
    boot-interrupts
+   controller/vmd
-- 
2.39.1