Documentation/s390/vfio-ccw.rst

   1 ==================================
   2 vfio-ccw: the basic infrastructure
   3 ==================================
   4
   5 Introduction
   6 ------------
   7
   8 Here we describe the vfio support for I/O subchannel devices for
   9 Linux/s390. Motivation for vfio-ccw is to passthrough subchannels to a
  10 virtual machine, while vfio is the means.
  11
  12 Different than other hardware architectures, s390 has defined a unified
  13 I/O access method, which is so called Channel I/O. It has its own access
  14 patterns:
  15
  16 - Channel programs run asynchronously on a separate (co)processor.
  17 - The channel subsystem will access any memory designated by the caller
  18   in the channel program directly, i.e. there is no iommu involved.
  19
  20 Thus when we introduce vfio support for these devices, we realize it
  21 with a mediated device (mdev) implementation. The vfio mdev will be
  22 added to an iommu group, so as to make itself able to be managed by the
  23 vfio framework. And we add read/write callbacks for special vfio I/O
  24 regions to pass the channel programs from the mdev to its parent device
  25 (the real I/O subchannel device) to do further address translation and
  26 to perform I/O instructions.
  27
  28 This document does not intend to explain the s390 I/O architecture in
  29 every detail. More information/reference could be found here:
  30
  31 - A good start to know Channel I/O in general:
  32   https://en.wikipedia.org/wiki/Channel_I/O
  33 - s390 architecture:
  34   s390 Principles of Operation manual (IBM Form. No. SA22-7832)
  35 - The existing QEMU code which implements a simple emulated channel
  36   subsystem could also be a good reference. It makes it easier to follow
  37   the flow.
  38   qemu/hw/s390x/css.c
  39
  40 For vfio mediated device framework:
  41 - Documentation/driver-api/vfio-mediated-device.rst
  42
  43 Motivation of vfio-ccw
  44 ----------------------
  45
  46 Typically, a guest virtualized via QEMU/KVM on s390 only sees
  47 paravirtualized virtio devices via the "Virtio Over Channel I/O
  48 (virtio-ccw)" transport. This makes virtio devices discoverable via
  49 standard operating system algorithms for handling channel devices.
  50
  51 However this is not enough. On s390 for the majority of devices, which
  52 use the standard Channel I/O based mechanism, we also need to provide
  53 the functionality of passing through them to a QEMU virtual machine.
  54 This includes devices that don't have a virtio counterpart (e.g. tape
  55 drives) or that have specific characteristics which guests want to
  56 exploit.
  57
  58 For passing a device to a guest, we want to use the same interface as
  59 everybody else, namely vfio. We implement this vfio support for channel
  60 devices via the vfio mediated device framework and the subchannel device
  61 driver "vfio_ccw".
  62
  63 Access patterns of CCW devices
  64 ------------------------------
  65
  66 s390 architecture has implemented a so called channel subsystem, that
  67 provides a unified view of the devices physically attached to the
  68 systems. Though the s390 hardware platform knows about a huge variety of
  69 different peripheral attachments like disk devices (aka. DASDs), tapes,
  70 communication controllers, etc. They can all be accessed by a well
  71 defined access method and they are presenting I/O completion a unified
  72 way: I/O interruptions.
  73
  74 All I/O requires the use of channel command words (CCWs). A CCW is an
  75 instruction to a specialized I/O channel processor. A channel program is
  76 a sequence of CCWs which are executed by the I/O channel subsystem.  To
  77 issue a channel program to the channel subsystem, it is required to
  78 build an operation request block (ORB), which can be used to point out
  79 the format of the CCW and other control information to the system. The
  80 operating system signals the I/O channel subsystem to begin executing
  81 the channel program with a SSCH (start sub-channel) instruction. The
  82 central processor is then free to proceed with non-I/O instructions
  83 until interrupted. The I/O completion result is received by the
  84 interrupt handler in the form of interrupt response block (IRB).
  85
  86 Back to vfio-ccw, in short:
  87
  88 - ORBs and channel programs are built in guest kernel (with guest
  89   physical addresses).
  90 - ORBs and channel programs are passed to the host kernel.
  91 - Host kernel translates the guest physical addresses to real addresses
  92   and starts the I/O with issuing a privileged Channel I/O instruction
  93   (e.g SSCH).
  94 - channel programs run asynchronously on a separate processor.
  95 - I/O completion will be signaled to the host with I/O interruptions.
  96   And it will be copied as IRB to user space to pass it back to the
  97   guest.
  98
  99 Physical vfio ccw device and its child mdev
 100 -------------------------------------------
 101
 102 As mentioned above, we realize vfio-ccw with a mdev implementation.
 103
 104 Channel I/O does not have IOMMU hardware support, so the physical
 105 vfio-ccw device does not have an IOMMU level translation or isolation.
 106
 107 Subchannel I/O instructions are all privileged instructions. When
 108 handling the I/O instruction interception, vfio-ccw has the software
 109 policing and translation how the channel program is programmed before
 110 it gets sent to hardware.
 111
 112 Within this implementation, we have two drivers for two types of
 113 devices:
 114
 115 - The vfio_ccw driver for the physical subchannel device.
 116   This is an I/O subchannel driver for the real subchannel device.  It
 117   realizes a group of callbacks and registers to the mdev framework as a
 118   parent (physical) device. As a consequence, mdev provides vfio_ccw a
 119   generic interface (sysfs) to create mdev devices. A vfio mdev could be
 120   created by vfio_ccw then and added to the mediated bus. It is the vfio
 121   device that added to an IOMMU group and a vfio group.
 122   vfio_ccw also provides an I/O region to accept channel program
 123   request from user space and store I/O interrupt result for user
 124   space to retrieve. To notify user space an I/O completion, it offers
 125   an interface to setup an eventfd fd for asynchronous signaling.
 126
 127 - The vfio_mdev driver for the mediated vfio ccw device.
 128   This is provided by the mdev framework. It is a vfio device driver for
 129   the mdev that created by vfio_ccw.
 130   It realizes a group of vfio device driver callbacks, adds itself to a
 131   vfio group, and registers itself to the mdev framework as a mdev
 132   driver.
 133   It uses a vfio iommu backend that uses the existing map and unmap
 134   ioctls, but rather than programming them into an IOMMU for a device,
 135   it simply stores the translations for use by later requests. This
 136   means that a device programmed in a VM with guest physical addresses
 137   can have the vfio kernel convert that address to process virtual
 138   address, pin the page and program the hardware with the host physical
 139   address in one step.
 140   For a mdev, the vfio iommu backend will not pin the pages during the
 141   VFIO_IOMMU_MAP_DMA ioctl. Mdev framework will only maintain a database
 142   of the iova<->vaddr mappings in this operation. And they export a
 143   vfio_pin_pages and a vfio_unpin_pages interfaces from the vfio iommu
 144   backend for the physical devices to pin and unpin pages by demand.
 145
 146 Below is a high Level block diagram::
 147
 148  +-------------+
 149  |             |
 150  | +---------+ | mdev_register_driver() +--------------+
 151  | |  Mdev   | +<-----------------------+              |
 152  | |  bus    | |                        | vfio_mdev.ko |
 153  | | driver  | +----------------------->+              |<-> VFIO user
 154  | +---------+ |    probe()/remove()    +--------------+    APIs
 155  |             |
 156  |  MDEV CORE  |
 157  |   MODULE    |
 158  |   mdev.ko   |
 159  | +---------+ | mdev_register_device() +--------------+
 160  | |Physical | +<-----------------------+              |
 161  | | device  | |                        |  vfio_ccw.ko |<-> subchannel
 162  | |interface| +----------------------->+              |     device
 163  | +---------+ |       callback         +--------------+
 164  +-------------+
 165
 166 The process of how these work together.
 167
 168 1. vfio_ccw.ko drives the physical I/O subchannel, and registers the
 169    physical device (with callbacks) to mdev framework.
 170    When vfio_ccw probing the subchannel device, it registers device
 171    pointer and callbacks to the mdev framework. Mdev related file nodes
 172    under the device node in sysfs would be created for the subchannel
 173    device, namely 'mdev_create', 'mdev_destroy' and
 174    'mdev_supported_types'.
 175 2. Create a mediated vfio ccw device.
 176    Use the 'mdev_create' sysfs file, we need to manually create one (and
 177    only one for our case) mediated device.
 178 3. vfio_mdev.ko drives the mediated ccw device.
 179    vfio_mdev is also the vfio device drvier. It will probe the mdev and
 180    add it to an iommu_group and a vfio_group. Then we could pass through
 181    the mdev to a guest.
 182
 183
 184 VFIO-CCW Regions
 185 ----------------
 186
 187 The vfio-ccw driver exposes MMIO regions to accept requests from and return
 188 results to userspace.
 189
 190 vfio-ccw I/O region
 191 -------------------
 192
 193 An I/O region is used to accept channel program request from user
 194 space and store I/O interrupt result for user space to retrieve. The
 195 definition of the region is::
 196
 197   struct ccw_io_region {
 198   #define ORB_AREA_SIZE 12
 199           __u8    orb_area[ORB_AREA_SIZE];
 200   #define SCSW_AREA_SIZE 12
 201           __u8    scsw_area[SCSW_AREA_SIZE];
 202   #define IRB_AREA_SIZE 96
 203           __u8    irb_area[IRB_AREA_SIZE];
 204           __u32   ret_code;
 205   } __packed;
 206
 207 This region is always available.
 208
 209 While starting an I/O request, orb_area should be filled with the
 210 guest ORB, and scsw_area should be filled with the SCSW of the Virtual
 211 Subchannel.
 212
 213 irb_area stores the I/O result.
 214
 215 ret_code stores a return code for each access of the region. The following
 216 values may occur:
 217
 218 ``0``
 219   The operation was successful.
 220
 221 ``-EOPNOTSUPP``
 222   The orb specified transport mode or an unidentified IDAW format, or the
 223   scsw specified a function other than the start function.
 224
 225 ``-EIO``
 226   A request was issued while the device was not in a state ready to accept
 227   requests, or an internal error occurred.
 228
 229 ``-EBUSY``
 230   The subchannel was status pending or busy, or a request is already active.
 231
 232 ``-EAGAIN``
 233   A request was being processed, and the caller should retry.
 234
 235 ``-EACCES``
 236   The channel path(s) used for the I/O were found to be not operational.
 237
 238 ``-ENODEV``
 239   The device was found to be not operational.
 240
 241 ``-EINVAL``
 242   The orb specified a chain longer than 255 ccws, or an internal error
 243   occurred.
 244
 245
 246 vfio-ccw cmd region
 247 -------------------
 248
 249 The vfio-ccw cmd region is used to accept asynchronous instructions
 250 from userspace::
 251
 252   #define VFIO_CCW_ASYNC_CMD_HSCH (1 << 0)
 253   #define VFIO_CCW_ASYNC_CMD_CSCH (1 << 1)
 254   struct ccw_cmd_region {
 255          __u32 command;
 256          __u32 ret_code;
 257   } __packed;
 258
 259 This region is exposed via region type VFIO_REGION_SUBTYPE_CCW_ASYNC_CMD.
 260
 261 Currently, CLEAR SUBCHANNEL and HALT SUBCHANNEL use this region.
 262
 263 command specifies the command to be issued; ret_code stores a return code
 264 for each access of the region. The following values may occur:
 265
 266 ``0``
 267   The operation was successful.
 268
 269 ``-ENODEV``
 270   The device was found to be not operational.
 271
 272 ``-EINVAL``
 273   A command other than halt or clear was specified.
 274
 275 ``-EIO``
 276   A request was issued while the device was not in a state ready to accept
 277   requests.
 278
 279 ``-EAGAIN``
 280   A request was being processed, and the caller should retry.
 281
 282 ``-EBUSY``
 283   The subchannel was status pending or busy while processing a halt request.
 284
 285 vfio-ccw schib region
 286 ---------------------
 287
 288 The vfio-ccw schib region is used to return Subchannel-Information
 289 Block (SCHIB) data to userspace::
 290
 291   struct ccw_schib_region {
 292   #define SCHIB_AREA_SIZE 52
 293          __u8 schib_area[SCHIB_AREA_SIZE];
 294   } __packed;
 295
 296 This region is exposed via region type VFIO_REGION_SUBTYPE_CCW_SCHIB.
 297
 298 Reading this region triggers a STORE SUBCHANNEL to be issued to the
 299 associated hardware.
 300
 301 vfio-ccw crw region
 302 ---------------------
 303
 304 The vfio-ccw crw region is used to return Channel Report Word (CRW)
 305 data to userspace::
 306
 307   struct ccw_crw_region {
 308          __u32 crw;
 309          __u32 pad;
 310   } __packed;
 311
 312 This region is exposed via region type VFIO_REGION_SUBTYPE_CCW_CRW.
 313
 314 Reading this region returns a CRW if one that is relevant for this
 315 subchannel (e.g. one reporting changes in channel path state) is
 316 pending, or all zeroes if not. If multiple CRWs are pending (including
 317 possibly chained CRWs), reading this region again will return the next
 318 one, until no more CRWs are pending and zeroes are returned. This is
 319 similar to how STORE CHANNEL REPORT WORD works.
 320
 321 vfio-ccw operation details
 322 --------------------------
 323
 324 vfio-ccw follows what vfio-pci did on the s390 platform and uses
 325 vfio-iommu-type1 as the vfio iommu backend.
 326
 327 * CCW translation APIs
 328   A group of APIs (start with `cp_`) to do CCW translation. The CCWs
 329   passed in by a user space program are organized with their guest
 330   physical memory addresses. These APIs will copy the CCWs into kernel
 331   space, and assemble a runnable kernel channel program by updating the
 332   guest physical addresses with their corresponding host physical addresses.
 333   Note that we have to use IDALs even for direct-access CCWs, as the
 334   referenced memory can be located anywhere, including above 2G.
 335
 336 * vfio_ccw device driver
 337   This driver utilizes the CCW translation APIs and introduces
 338   vfio_ccw, which is the driver for the I/O subchannel devices you want
 339   to pass through.
 340   vfio_ccw implements the following vfio ioctls::
 341
 342     VFIO_DEVICE_GET_INFO
 343     VFIO_DEVICE_GET_IRQ_INFO
 344     VFIO_DEVICE_GET_REGION_INFO
 345     VFIO_DEVICE_RESET
 346     VFIO_DEVICE_SET_IRQS
 347
 348   This provides an I/O region, so that the user space program can pass a
 349   channel program to the kernel, to do further CCW translation before
 350   issuing them to a real device.
 351   This also provides the SET_IRQ ioctl to setup an event notifier to
 352   notify the user space program the I/O completion in an asynchronous
 353   way.
 354
 355 The use of vfio-ccw is not limited to QEMU, while QEMU is definitely a
 356 good example to get understand how these patches work. Here is a little
 357 bit more detail how an I/O request triggered by the QEMU guest will be
 358 handled (without error handling).
 359
 360 Explanation:
 361
 362 - Q1-Q7: QEMU side process.
 363 - K1-K5: Kernel side process.
 364
 365 Q1.
 366     Get I/O region info during initialization.
 367
 368 Q2.
 369     Setup event notifier and handler to handle I/O completion.
 370
 371 ... ...
 372
 373 Q3.
 374     Intercept a ssch instruction.
 375 Q4.
 376     Write the guest channel program and ORB to the I/O region.
 377
 378     K1.
 379         Copy from guest to kernel.
 380     K2.
 381         Translate the guest channel program to a host kernel space
 382         channel program, which becomes runnable for a real device.
 383     K3.
 384         With the necessary information contained in the orb passed in
 385         by QEMU, issue the ccwchain to the device.
 386     K4.
 387         Return the ssch CC code.
 388 Q5.
 389     Return the CC code to the guest.
 390
 391 ... ...
 392
 393     K5.
 394         Interrupt handler gets the I/O result and write the result to
 395         the I/O region.
 396     K6.
 397         Signal QEMU to retrieve the result.
 398
 399 Q6.
 400     Get the signal and event handler reads out the result from the I/O
 401     region.
 402 Q7.
 403     Update the irb for the guest.
 404
 405 Limitations
 406 -----------
 407
 408 The current vfio-ccw implementation focuses on supporting basic commands
 409 needed to implement block device functionality (read/write) of DASD/ECKD
 410 device only. Some commands may need special handling in the future, for
 411 example, anything related to path grouping.
 412
 413 DASD is a kind of storage device. While ECKD is a data recording format.
 414 More information for DASD and ECKD could be found here:
 415 https://en.wikipedia.org/wiki/Direct-access_storage_device
 416 https://en.wikipedia.org/wiki/Count_key_data
 417
 418 Together with the corresponding work in QEMU, we can bring the passed
 419 through DASD/ECKD device online in a guest now and use it as a block
 420 device.
 421
 422 The current code allows the guest to start channel programs via
 423 START SUBCHANNEL, and to issue HALT SUBCHANNEL, CLEAR SUBCHANNEL,
 424 and STORE SUBCHANNEL.
 425
 426 Currently all channel programs are prefetched, regardless of the
 427 p-bit setting in the ORB.  As a result, self modifying channel
 428 programs are not supported.  For this reason, IPL has to be handled as
 429 a special case by a userspace/guest program; this has been implemented
 430 in QEMU's s390-ccw bios as of QEMU 4.1.
 431
 432 vfio-ccw supports classic (command mode) channel I/O only. Transport
 433 mode (HPF) is not supported.
 434
 435 QDIO subchannels are currently not supported. Classic devices other than
 436 DASD/ECKD might work, but have not been tested.
 437
 438 Reference
 439 ---------
 440 1. ESA/s390 Principles of Operation manual (IBM Form. No. SA22-7832)
 441 2. ESA/390 Common I/O Device Commands manual (IBM Form. No. SA22-7204)
 442 3. https://en.wikipedia.org/wiki/Channel_I/O
 443 4. Documentation/s390/cds.rst
 444 5. Documentation/driver-api/vfio.rst
 445 6. Documentation/driver-api/vfio-mediated-device.rst