--- /dev/null
+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
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