docs: add documentation for vfio-ccw

	- HiperSockets Bridge Port Support.

	- HiperSockets Bridge Port Support.

s390dbf.txt

s390dbf.txt

	- information on using the s390 debug feature.

	- information on using the s390 debug feature.

vfio-ccw.txt

	  information on the vfio-ccw I/O subchannel driver.

zfcpdump.txt

zfcpdump.txt

	- information on the s390 SCSI dump tool.

	- information on the s390 SCSI dump tool.

vfio-ccw: the basic infrastructure

==================================

Introduction

------------

Here we describe the vfio support for I/O subchannel devices for

Linux/s390. Motivation for vfio-ccw is to passthrough subchannels to a

virtual machine, while vfio is the means.

Different than other hardware architectures, s390 has defined a unified

I/O access method, which is so called Channel I/O. It has its own access

patterns:

- Channel programs run asynchronously on a separate (co)processor.

- The channel subsystem will access any memory designated by the caller

  in the channel program directly, i.e. there is no iommu involved.

Thus when we introduce vfio support for these devices, we realize it

with a mediated device (mdev) implementation. The vfio mdev will be

added to an iommu group, so as to make itself able to be managed by the

vfio framework. And we add read/write callbacks for special vfio I/O

regions to pass the channel programs from the mdev to its parent device

(the real I/O subchannel device) to do further address translation and

to perform I/O instructions.

This document does not intend to explain the s390 I/O architecture in

every detail. More information/reference could be found here:

- A good start to know Channel I/O in general:

  https://en.wikipedia.org/wiki/Channel_I/O

- s390 architecture:

  s390 Principles of Operation manual (IBM Form. No. SA22-7832)

- The existing Qemu code which implements a simple emulated channel

  subsystem could also be a good reference. It makes it easier to follow

  the flow.

  qemu/hw/s390x/css.c

For vfio mediated device framework:

- Documentation/vfio-mediated-device.txt

Motivation of vfio-ccw

----------------------

Currently, a guest virtualized via qemu/kvm on s390 only sees

paravirtualized virtio devices via the "Virtio Over Channel I/O

(virtio-ccw)" transport. This makes virtio devices discoverable via

standard operating system algorithms for handling channel devices.

However this is not enough. On s390 for the majority of devices, which

use the standard Channel I/O based mechanism, we also need to provide

the functionality of passing through them to a Qemu virtual machine.

This includes devices that don't have a virtio counterpart (e.g. tape

drives) or that have specific characteristics which guests want to

exploit.

For passing a device to a guest, we want to use the same interface as

everybody else, namely vfio. Thus, we would like to introduce vfio

support for channel devices. And we would like to name this new vfio

device "vfio-ccw".

Access patterns of CCW devices

------------------------------

s390 architecture has implemented a so called channel subsystem, that

provides a unified view of the devices physically attached to the

systems. Though the s390 hardware platform knows about a huge variety of

different peripheral attachments like disk devices (aka. DASDs), tapes,

communication controllers, etc. They can all be accessed by a well

defined access method and they are presenting I/O completion a unified

way: I/O interruptions.

All I/O requires the use of channel command words (CCWs). A CCW is an

instruction to a specialized I/O channel processor. A channel program is

a sequence of CCWs which are executed by the I/O channel subsystem.  To

issue a channel program to the channel subsystem, it is required to

build an operation request block (ORB), which can be used to point out

the format of the CCW and other control information to the system. The

operating system signals the I/O channel subsystem to begin executing

the channel program with a SSCH (start sub-channel) instruction. The

central processor is then free to proceed with non-I/O instructions

until interrupted. The I/O completion result is received by the

interrupt handler in the form of interrupt response block (IRB).

Back to vfio-ccw, in short:

- ORBs and channel programs are built in guest kernel (with guest

  physical addresses).

- ORBs and channel programs are passed to the host kernel.

- Host kernel translates the guest physical addresses to real addresses

  and starts the I/O with issuing a privileged Channel I/O instruction

  (e.g SSCH).

- channel programs run asynchronously on a separate processor.

- I/O completion will be signaled to the host with I/O interruptions.

  And it will be copied as IRB to user space to pass it back to the

  guest.

Physical vfio ccw device and its child mdev

-------------------------------------------

As mentioned above, we realize vfio-ccw with a mdev implementation.

Channel I/O does not have IOMMU hardware support, so the physical

vfio-ccw device does not have an IOMMU level translation or isolation.

Sub-channel I/O instructions are all privileged instructions, When

handling the I/O instruction interception, vfio-ccw has the software

policing and translation how the channel program is programmed before

it gets sent to hardware.

Within this implementation, we have two drivers for two types of

devices:

- The vfio_ccw driver for the physical subchannel device.

  This is an I/O subchannel driver for the real subchannel device.  It

  realizes a group of callbacks and registers to the mdev framework as a

  parent (physical) device. As a consequence, mdev provides vfio_ccw a

  generic interface (sysfs) to create mdev devices. A vfio mdev could be

  created by vfio_ccw then and added to the mediated bus. It is the vfio

  device that added to an IOMMU group and a vfio group.

  vfio_ccw also provides an I/O region to accept channel program

  request from user space and store I/O interrupt result for user

  space to retrieve. To notify user space an I/O completion, it offers

  an interface to setup an eventfd fd for asynchronous signaling.

- The vfio_mdev driver for the mediated vfio ccw device.

  This is provided by the mdev framework. It is a vfio device driver for

  the mdev that created by vfio_ccw.

  It realize a group of vfio device driver callbacks, adds itself to a

  vfio group, and registers itself to the mdev framework as a mdev

  driver.

  It uses a vfio iommu backend that uses the existing map and unmap

  ioctls, but rather than programming them into an IOMMU for a device,

  it simply stores the translations for use by later requests. This

  means that a device programmed in a VM with guest physical addresses

  can have the vfio kernel convert that address to process virtual

  address, pin the page and program the hardware with the host physical

  address in one step.

  For a mdev, the vfio iommu backend will not pin the pages during the

  VFIO_IOMMU_MAP_DMA ioctl. Mdev framework will only maintain a database

  of the iova<->vaddr mappings in this operation. And they export a

  vfio_pin_pages and a vfio_unpin_pages interfaces from the vfio iommu

  backend for the physical devices to pin and unpin pages by demand.

Below is a high Level block diagram.

 +-------------+

 |             |

 | +---------+ | mdev_register_driver() +--------------+

 | |  Mdev   | +<-----------------------+              |

 | |  bus    | |                        | vfio_mdev.ko |

 | | driver  | +----------------------->+              |<-> VFIO user

 | +---------+ |    probe()/remove()    +--------------+    APIs

 |             |

 |  MDEV CORE  |

 |   MODULE    |

 |   mdev.ko   |

 | +---------+ | mdev_register_device() +--------------+

 | |Physical | +<-----------------------+              |

 | | device  | |                        |  vfio_ccw.ko |<-> subchannel

 | |interface| +----------------------->+              |     device

 | +---------+ |       callback         +--------------+

 +-------------+

The process of how these work together.

1. vfio_ccw.ko drives the physical I/O subchannel, and registers the

   physical device (with callbacks) to mdev framework.

   When vfio_ccw probing the subchannel device, it registers device

   pointer and callbacks to the mdev framework. Mdev related file nodes

   under the device node in sysfs would be created for the subchannel

   device, namely 'mdev_create', 'mdev_destroy' and

   'mdev_supported_types'.

2. Create a mediated vfio ccw device.

   Use the 'mdev_create' sysfs file, we need to manually create one (and

   only one for our case) mediated device.

3. vfio_mdev.ko drives the mediated ccw device.

   vfio_mdev is also the vfio device drvier. It will probe the mdev and

   add it to an iommu_group and a vfio_group. Then we could pass through

   the mdev to a guest.

vfio-ccw I/O region

-------------------

An I/O region is used to accept channel program request from user

space and store I/O interrupt result for user space to retrieve. The

defination of the region is:

struct ccw_io_region {

#define ORB_AREA_SIZE 12

	__u8	orb_area[ORB_AREA_SIZE];

#define SCSW_AREA_SIZE 12

	__u8	scsw_area[SCSW_AREA_SIZE];

#define IRB_AREA_SIZE 96

	__u8	irb_area[IRB_AREA_SIZE];

	__u32	ret_code;

} __packed;

While starting an I/O request, orb_area should be filled with the

guest ORB, and scsw_area should be filled with the SCSW of the Virtual

Subchannel.

irb_area stores the I/O result.

ret_code stores a return code for each access of the region.

vfio-ccw patches overview

-------------------------

For now, our patches are rebased on the latest mdev implementation.

vfio-ccw follows what vfio-pci did on the s390 paltform and uses

vfio-iommu-type1 as the vfio iommu backend. It's a good start to launch

the code review for vfio-ccw. Note that the implementation is far from

complete yet; but we'd like to get feedback for the general

architecture.

* CCW translation APIs

- Description:

  These introduce a group of APIs (start with 'cp_') to do CCW

  translation. The CCWs passed in by a user space program are

  organized with their guest physical memory addresses. These APIs

  will copy the CCWs into the kernel space, and assemble a runnable

  kernel channel program by updating the guest physical addresses with

  their corresponding host physical addresses.

- Patches:

  vfio: ccw: introduce channel program interfaces

* vfio_ccw device driver

- Description:

  The following patches utilizes the CCW translation APIs and introduce

  vfio_ccw, which is the driver for the I/O subchannel devices you want

  to pass through.

  vfio_ccw implements the following vfio ioctls:

    VFIO_DEVICE_GET_INFO

    VFIO_DEVICE_GET_IRQ_INFO

    VFIO_DEVICE_GET_REGION_INFO

    VFIO_DEVICE_RESET

    VFIO_DEVICE_SET_IRQS

  This provides an I/O region, so that the user space program can pass a

  channel program to the kernel, to do further CCW translation before

  issuing them to a real device.

  This also provides the SET_IRQ ioctl to setup an event notifier to

  notify the user space program the I/O completion in an asynchronous

  way.

- Patches:

  vfio: ccw: basic implementation for vfio_ccw driver

  vfio: ccw: introduce ccw_io_region

  vfio: ccw: realize VFIO_DEVICE_GET_REGION_INFO ioctl

  vfio: ccw: realize VFIO_DEVICE_RESET ioctl

  vfio: ccw: realize VFIO_DEVICE_G(S)ET_IRQ_INFO ioctls

The user of vfio-ccw is not limited to Qemu, while Qemu is definitely a

good example to get understand how these patches work. Here is a little

bit more detail how an I/O request triggered by the Qemu guest will be

handled (without error handling).

Explanation:

Q1-Q7: Qemu side process.

K1-K5: Kernel side process.

Q1. Get I/O region info during initialization.

Q2. Setup event notifier and handler to handle I/O completion.

... ...

Q3. Intercept a ssch instruction.

Q4. Write the guest channel program and ORB to the I/O region.

    K1. Copy from guest to kernel.

    K2. Translate the guest channel program to a host kernel space

        channel program, which becomes runnable for a real device.

    K3. With the necessary information contained in the orb passed in

        by Qemu, issue the ccwchain to the device.

    K4. Return the ssch CC code.

Q5. Return the CC code to the guest.

... ...

    K5. Interrupt handler gets the I/O result and write the result to

        the I/O region.

    K6. Signal Qemu to retrieve the result.

Q6. Get the signal and event handler reads out the result from the I/O

    region.

Q7. Update the irb for the guest.

Limitations

-----------

The current vfio-ccw implementation focuses on supporting basic commands

needed to implement block device functionality (read/write) of DASD/ECKD

device only. Some commands may need special handling in the future, for

example, anything related to path grouping.

DASD is a kind of storage device. While ECKD is a data recording format.

More information for DASD and ECKD could be found here:

https://en.wikipedia.org/wiki/Direct-access_storage_device

https://en.wikipedia.org/wiki/Count_key_data

Together with the corresponding work in Qemu, we can bring the passed

through DASD/ECKD device online in a guest now and use it as a block

device.

Reference

---------

1. ESA/s390 Principles of Operation manual (IBM Form. No. SA22-7832)

2. ESA/390 Common I/O Device Commands manual (IBM Form. No. SA22-7204)

3. https://en.wikipedia.org/wiki/Channel_I/O

4. Documentation/s390/cds.txt

5. Documentation/vfio.txt

6. Documentation/vfio-mediated-device.txt