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linux-next/Documentation/vfio-mediated-device.txt
Alex Williamson 002fe996f6 vfio/mdev: Check globally for duplicate devices 
When we create an mdev device, we check for duplicates against the
parent device and return -EEXIST if found, but the mdev device
namespace is global since we'll link all devices from the bus.  We do
catch this later in sysfs_do_create_link_sd() to return -EEXIST, but
with it comes a kernel warning and stack trace for trying to create
duplicate sysfs links, which makes it an undesirable response.

Therefore we should really be looking for duplicates across all mdev
parent devices, or as implemented here, against our mdev device list.
Using mdev_list to prevent duplicates means that we can remove
mdev_parent.lock, but in order not to serialize mdev device creation
and removal globally, we add mdev_device.active which allows UUIDs to
be reserved such that we can drop the mdev_list_lock before the mdev
device is fully in place.

Two behavioral notes; first, mdev_parent.lock had the side-effect of
serializing mdev create and remove ops per parent device.  This was
an implementation detail, not an intentional guarantee provided to
the mdev vendor drivers.  Vendor drivers can trivially provide this
serialization internally if necessary.  Second, review comments note
the new -EAGAIN behavior when the device, and in particular the remove
attribute, becomes visible in sysfs.  If a remove is triggered prior
to completion of mdev_device_create() the user will see a -EAGAIN
error.  While the errno is different, receiving an error during this
period is not, the previous implementation returned -ENODEV for the
same condition.  Furthermore, the consistency to the user is improved
in the case where mdev_device_remove_ops() returns error.  Previously
concurrent calls to mdev_device_remove() could see the device
disappear with -ENODEV and return in the case of error.  Now a user
would see -EAGAIN while the device is in this transitory state.

Reviewed-by: Kirti Wankhede <kwankhede@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
2018-06-08 10:24:27 -06:00

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.. include:: <isonum.txt>
=====================
VFIO Mediated devices
=====================
:Copyright: |copy| 2016, NVIDIA CORPORATION. All rights reserved.
:Author: Neo Jia <cjia@nvidia.com>
:Author: Kirti Wankhede <kwankhede@nvidia.com>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License version 2 as
published by the Free Software Foundation.
Virtual Function I/O (VFIO) Mediated devices[1]
===============================================
The number of use cases for virtualizing DMA devices that do not have built-in
SR_IOV capability is increasing. Previously, to virtualize such devices,
developers had to create their own management interfaces and APIs, and then
integrate them with user space software. To simplify integration with user space
software, we have identified common requirements and a unified management
interface for such devices.
The VFIO driver framework provides unified APIs for direct device access. It is
an IOMMU/device-agnostic framework for exposing direct device access to user
space in a secure, IOMMU-protected environment. This framework is used for
multiple devices, such as GPUs, network adapters, and compute accelerators. With
direct device access, virtual machines or user space applications have direct
access to the physical device. This framework is reused for mediated devices.
The mediated core driver provides a common interface for mediated device
management that can be used by drivers of different devices. This module
provides a generic interface to perform these operations:
* Create and destroy a mediated device
* Add a mediated device to and remove it from a mediated bus driver
* Add a mediated device to and remove it from an IOMMU group
The mediated core driver also provides an interface to register a bus driver.
For example, the mediated VFIO mdev driver is designed for mediated devices and
supports VFIO APIs. The mediated bus driver adds a mediated device to and
removes it from a VFIO group.
The following high-level block diagram shows the main components and interfaces
in the VFIO mediated driver framework. The diagram shows NVIDIA, Intel, and IBM
devices as examples, as these devices are the first devices to use this module::
+---------------+
| |
| +-----------+ | mdev_register_driver() +--------------+
| | | +<------------------------+ |
| | mdev | | | |
| | bus | +------------------------>+ vfio_mdev.ko |<-> VFIO user
| | driver | | probe()/remove() | | APIs
| | | | +--------------+
| +-----------+ |
| |
| MDEV CORE |
| MODULE |
| mdev.ko |
| +-----------+ | mdev_register_device() +--------------+
| | | +<------------------------+ |
| | | | | nvidia.ko |<-> physical
| | | +------------------------>+ | device
| | | | callbacks +--------------+
| | Physical | |
| | device | | mdev_register_device() +--------------+
| | interface | |<------------------------+ |
| | | | | i915.ko |<-> physical
| | | +------------------------>+ | device
| | | | callbacks +--------------+
| | | |
| | | | mdev_register_device() +--------------+
| | | +<------------------------+ |
| | | | | ccw_device.ko|<-> physical
| | | +------------------------>+ | device
| | | | callbacks +--------------+
| +-----------+ |
+---------------+
Registration Interfaces
=======================
The mediated core driver provides the following types of registration
interfaces:
* Registration interface for a mediated bus driver
* Physical device driver interface
Registration Interface for a Mediated Bus Driver
------------------------------------------------
The registration interface for a mediated bus driver provides the following
structure to represent a mediated device's driver::
/*
* struct mdev_driver [2] - Mediated device's driver
* @name: driver name
* @probe: called when new device created
* @remove: called when device removed
* @driver: device driver structure
*/
struct mdev_driver {
const char *name;
int (*probe) (struct device *dev);
void (*remove) (struct device *dev);
struct device_driver driver;
};
A mediated bus driver for mdev should use this structure in the function calls
to register and unregister itself with the core driver:
* Register::
extern int mdev_register_driver(struct mdev_driver *drv,
struct module *owner);
* Unregister::
extern void mdev_unregister_driver(struct mdev_driver *drv);
The mediated bus driver is responsible for adding mediated devices to the VFIO
group when devices are bound to the driver and removing mediated devices from
the VFIO when devices are unbound from the driver.
Physical Device Driver Interface
--------------------------------
The physical device driver interface provides the mdev_parent_ops[3] structure
to define the APIs to manage work in the mediated core driver that is related
to the physical device.
The structures in the mdev_parent_ops structure are as follows:
* dev_attr_groups: attributes of the parent device
* mdev_attr_groups: attributes of the mediated device
* supported_config: attributes to define supported configurations
The functions in the mdev_parent_ops structure are as follows:
* create: allocate basic resources in a driver for a mediated device
* remove: free resources in a driver when a mediated device is destroyed
(Note that mdev-core provides no implicit serialization of create/remove
callbacks per mdev parent device, per mdev type, or any other categorization.
Vendor drivers are expected to be fully asynchronous in this respect or
provide their own internal resource protection.)
The callbacks in the mdev_parent_ops structure are as follows:
* open: open callback of mediated device
* close: close callback of mediated device
* ioctl: ioctl callback of mediated device
* read : read emulation callback
* write: write emulation callback
* mmap: mmap emulation callback
A driver should use the mdev_parent_ops structure in the function call to
register itself with the mdev core driver::
extern int mdev_register_device(struct device *dev,
const struct mdev_parent_ops *ops);
However, the mdev_parent_ops structure is not required in the function call
that a driver should use to unregister itself with the mdev core driver::
extern void mdev_unregister_device(struct device *dev);
Mediated Device Management Interface Through sysfs
==================================================
The management interface through sysfs enables user space software, such as
libvirt, to query and configure mediated devices in a hardware-agnostic fashion.
This management interface provides flexibility to the underlying physical
device's driver to support features such as:
* Mediated device hot plug
* Multiple mediated devices in a single virtual machine
* Multiple mediated devices from different physical devices
Links in the mdev_bus Class Directory
-------------------------------------
The /sys/class/mdev_bus/ directory contains links to devices that are registered
with the mdev core driver.
Directories and files under the sysfs for Each Physical Device
--------------------------------------------------------------
::
|- [parent physical device]
|--- Vendor-specific-attributes [optional]
|--- [mdev_supported_types]
| |--- [<type-id>]
| | |--- create
| | |--- name
| | |--- available_instances
| | |--- device_api
| | |--- description
| | |--- [devices]
| |--- [<type-id>]
| | |--- create
| | |--- name
| | |--- available_instances
| | |--- device_api
| | |--- description
| | |--- [devices]
| |--- [<type-id>]
| |--- create
| |--- name
| |--- available_instances
| |--- device_api
| |--- description
| |--- [devices]
* [mdev_supported_types]
The list of currently supported mediated device types and their details.
[<type-id>], device_api, and available_instances are mandatory attributes
that should be provided by vendor driver.
* [<type-id>]
The [<type-id>] name is created by adding the device driver string as a prefix
to the string provided by the vendor driver. This format of this name is as
follows::
sprintf(buf, "%s-%s", dev_driver_string(parent->dev), group->name);
(or using mdev_parent_dev(mdev) to arrive at the parent device outside
of the core mdev code)
* device_api
This attribute should show which device API is being created, for example,
"vfio-pci" for a PCI device.
* available_instances
This attribute should show the number of devices of type <type-id> that can be
created.
* [device]
This directory contains links to the devices of type <type-id> that have been
created.
* name
This attribute should show human readable name. This is optional attribute.
* description
This attribute should show brief features/description of the type. This is
optional attribute.
Directories and Files Under the sysfs for Each mdev Device
----------------------------------------------------------
::
|- [parent phy device]
|--- [$MDEV_UUID]
|--- remove
|--- mdev_type {link to its type}
|--- vendor-specific-attributes [optional]
* remove (write only)
Writing '1' to the 'remove' file destroys the mdev device. The vendor driver can
fail the remove() callback if that device is active and the vendor driver
doesn't support hot unplug.
Example::
# echo 1 > /sys/bus/mdev/devices/$mdev_UUID/remove
Mediated device Hot plug
------------------------
Mediated devices can be created and assigned at runtime. The procedure to hot
plug a mediated device is the same as the procedure to hot plug a PCI device.
Translation APIs for Mediated Devices
=====================================
The following APIs are provided for translating user pfn to host pfn in a VFIO
driver::
extern int vfio_pin_pages(struct device *dev, unsigned long *user_pfn,
int npage, int prot, unsigned long *phys_pfn);
extern int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn,
int npage);
These functions call back into the back-end IOMMU module by using the pin_pages
and unpin_pages callbacks of the struct vfio_iommu_driver_ops[4]. Currently
these callbacks are supported in the TYPE1 IOMMU module. To enable them for
other IOMMU backend modules, such as PPC64 sPAPR module, they need to provide
these two callback functions.
Using the Sample Code
=====================
mtty.c in samples/vfio-mdev/ directory is a sample driver program to
demonstrate how to use the mediated device framework.
The sample driver creates an mdev device that simulates a serial port over a PCI
card.
1. Build and load the mtty.ko module.
This step creates a dummy device, /sys/devices/virtual/mtty/mtty/
Files in this device directory in sysfs are similar to the following::
# tree /sys/devices/virtual/mtty/mtty/
/sys/devices/virtual/mtty/mtty/
|-- mdev_supported_types
| |-- mtty-1
| | |-- available_instances
| | |-- create
| | |-- device_api
| | |-- devices
| | `-- name
| `-- mtty-2
| |-- available_instances
| |-- create
| |-- device_api
| |-- devices
| `-- name
|-- mtty_dev
| `-- sample_mtty_dev
|-- power
| |-- autosuspend_delay_ms
| |-- control
| |-- runtime_active_time
| |-- runtime_status
| `-- runtime_suspended_time
|-- subsystem -> ../../../../class/mtty
`-- uevent
2. Create a mediated device by using the dummy device that you created in the
previous step::
# echo "83b8f4f2-509f-382f-3c1e-e6bfe0fa1001" > \
/sys/devices/virtual/mtty/mtty/mdev_supported_types/mtty-2/create
3. Add parameters to qemu-kvm::
-device vfio-pci,\
sysfsdev=/sys/bus/mdev/devices/83b8f4f2-509f-382f-3c1e-e6bfe0fa1001
4. Boot the VM.
In the Linux guest VM, with no hardware on the host, the device appears
as follows::
# lspci -s 00:05.0 -xxvv
00:05.0 Serial controller: Device 4348:3253 (rev 10) (prog-if 02 [16550])
Subsystem: Device 4348:3253
Physical Slot: 5
Control: I/O+ Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr-
Stepping- SERR- FastB2B- DisINTx-
Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=medium >TAbort-
<TAbort- <MAbort- >SERR- <PERR- INTx-
Interrupt: pin A routed to IRQ 10
Region 0: I/O ports at c150 [size=8]
Region 1: I/O ports at c158 [size=8]
Kernel driver in use: serial
00: 48 43 53 32 01 00 00 02 10 02 00 07 00 00 00 00
10: 51 c1 00 00 59 c1 00 00 00 00 00 00 00 00 00 00
20: 00 00 00 00 00 00 00 00 00 00 00 00 48 43 53 32
30: 00 00 00 00 00 00 00 00 00 00 00 00 0a 01 00 00
In the Linux guest VM, dmesg output for the device is as follows:
serial 0000:00:05.0: PCI INT A -> Link[LNKA] -> GSI 10 (level, high) -> IRQ 10
0000:00:05.0: ttyS1 at I/O 0xc150 (irq = 10) is a 16550A
0000:00:05.0: ttyS2 at I/O 0xc158 (irq = 10) is a 16550A
5. In the Linux guest VM, check the serial ports::
# setserial -g /dev/ttyS*
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
/dev/ttyS1, UART: 16550A, Port: 0xc150, IRQ: 10
/dev/ttyS2, UART: 16550A, Port: 0xc158, IRQ: 10
6. Using minicom or any terminal emulation program, open port /dev/ttyS1 or
/dev/ttyS2 with hardware flow control disabled.
7. Type data on the minicom terminal or send data to the terminal emulation
program and read the data.
Data is loop backed from hosts mtty driver.
8. Destroy the mediated device that you created::
# echo 1 > /sys/bus/mdev/devices/83b8f4f2-509f-382f-3c1e-e6bfe0fa1001/remove
References
==========
1. See Documentation/vfio.txt for more information on VFIO.
2. struct mdev_driver in include/linux/mdev.h
3. struct mdev_parent_ops in include/linux/mdev.h
4. struct vfio_iommu_driver_ops in include/linux/vfio.h
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