358 lines
18 KiB
Plaintext
358 lines
18 KiB
Plaintext
Introduction
|
|
============
|
|
|
|
The IBM Power architecture provides support for CAPI (Coherent
|
|
Accelerator Power Interface), which is available to certain PCIe slots
|
|
on Power 8 systems. CAPI can be thought of as a special tunneling
|
|
protocol through PCIe that allow PCIe adapters to look like special
|
|
purpose co-processors which can read or write an application's
|
|
memory and generate page faults. As a result, the host interface to
|
|
an adapter running in CAPI mode does not require the data buffers to
|
|
be mapped to the device's memory (IOMMU bypass) nor does it require
|
|
memory to be pinned.
|
|
|
|
On Linux, Coherent Accelerator (CXL) kernel services present CAPI
|
|
devices as a PCI device by implementing a virtual PCI host bridge.
|
|
This abstraction simplifies the infrastructure and programming
|
|
model, allowing for drivers to look similar to other native PCI
|
|
device drivers.
|
|
|
|
CXL provides a mechanism by which user space applications can
|
|
directly talk to a device (network or storage) bypassing the typical
|
|
kernel/device driver stack. The CXL Flash Adapter Driver enables a
|
|
user space application direct access to Flash storage.
|
|
|
|
The CXL Flash Adapter Driver is a kernel module that sits in the
|
|
SCSI stack as a low level device driver (below the SCSI disk and
|
|
protocol drivers) for the IBM CXL Flash Adapter. This driver is
|
|
responsible for the initialization of the adapter, setting up the
|
|
special path for user space access, and performing error recovery. It
|
|
communicates directly the Flash Accelerator Functional Unit (AFU)
|
|
as described in Documentation/powerpc/cxl.txt.
|
|
|
|
The cxlflash driver supports two, mutually exclusive, modes of
|
|
operation at the device (LUN) level:
|
|
|
|
- Any flash device (LUN) can be configured to be accessed as a
|
|
regular disk device (i.e.: /dev/sdc). This is the default mode.
|
|
|
|
- Any flash device (LUN) can be configured to be accessed from
|
|
user space with a special block library. This mode further
|
|
specifies the means of accessing the device and provides for
|
|
either raw access to the entire LUN (referred to as direct
|
|
or physical LUN access) or access to a kernel/AFU-mediated
|
|
partition of the LUN (referred to as virtual LUN access). The
|
|
segmentation of a disk device into virtual LUNs is assisted
|
|
by special translation services provided by the Flash AFU.
|
|
|
|
Overview
|
|
========
|
|
|
|
The Coherent Accelerator Interface Architecture (CAIA) introduces a
|
|
concept of a master context. A master typically has special privileges
|
|
granted to it by the kernel or hypervisor allowing it to perform AFU
|
|
wide management and control. The master may or may not be involved
|
|
directly in each user I/O, but at the minimum is involved in the
|
|
initial setup before the user application is allowed to send requests
|
|
directly to the AFU.
|
|
|
|
The CXL Flash Adapter Driver establishes a master context with the
|
|
AFU. It uses memory mapped I/O (MMIO) for this control and setup. The
|
|
Adapter Problem Space Memory Map looks like this:
|
|
|
|
+-------------------------------+
|
|
| 512 * 64 KB User MMIO |
|
|
| (per context) |
|
|
| User Accessible |
|
|
+-------------------------------+
|
|
| 512 * 128 B per context |
|
|
| Provisioning and Control |
|
|
| Trusted Process accessible |
|
|
+-------------------------------+
|
|
| 64 KB Global |
|
|
| Trusted Process accessible |
|
|
+-------------------------------+
|
|
|
|
This driver configures itself into the SCSI software stack as an
|
|
adapter driver. The driver is the only entity that is considered a
|
|
Trusted Process to program the Provisioning and Control and Global
|
|
areas in the MMIO Space shown above. The master context driver
|
|
discovers all LUNs attached to the CXL Flash adapter and instantiates
|
|
scsi block devices (/dev/sdb, /dev/sdc etc.) for each unique LUN
|
|
seen from each path.
|
|
|
|
Once these scsi block devices are instantiated, an application
|
|
written to a specification provided by the block library may get
|
|
access to the Flash from user space (without requiring a system call).
|
|
|
|
This master context driver also provides a series of ioctls for this
|
|
block library to enable this user space access. The driver supports
|
|
two modes for accessing the block device.
|
|
|
|
The first mode is called a virtual mode. In this mode a single scsi
|
|
block device (/dev/sdb) may be carved up into any number of distinct
|
|
virtual LUNs. The virtual LUNs may be resized as long as the sum of
|
|
the sizes of all the virtual LUNs, along with the meta-data associated
|
|
with it does not exceed the physical capacity.
|
|
|
|
The second mode is called the physical mode. In this mode a single
|
|
block device (/dev/sdb) may be opened directly by the block library
|
|
and the entire space for the LUN is available to the application.
|
|
|
|
Only the physical mode provides persistence of the data. i.e. The
|
|
data written to the block device will survive application exit and
|
|
restart and also reboot. The virtual LUNs do not persist (i.e. do
|
|
not survive after the application terminates or the system reboots).
|
|
|
|
|
|
Block library API
|
|
=================
|
|
|
|
Applications intending to get access to the CXL Flash from user
|
|
space should use the block library, as it abstracts the details of
|
|
interfacing directly with the cxlflash driver that are necessary for
|
|
performing administrative actions (i.e.: setup, tear down, resize).
|
|
The block library can be thought of as a 'user' of services,
|
|
implemented as IOCTLs, that are provided by the cxlflash driver
|
|
specifically for devices (LUNs) operating in user space access
|
|
mode. While it is not a requirement that applications understand
|
|
the interface between the block library and the cxlflash driver,
|
|
a high-level overview of each supported service (IOCTL) is provided
|
|
below.
|
|
|
|
The block library can be found on GitHub:
|
|
http://github.com/open-power/capiflash
|
|
|
|
|
|
CXL Flash Driver IOCTLs
|
|
=======================
|
|
|
|
Users, such as the block library, that wish to interface with a flash
|
|
device (LUN) via user space access need to use the services provided
|
|
by the cxlflash driver. As these services are implemented as ioctls,
|
|
a file descriptor handle must first be obtained in order to establish
|
|
the communication channel between a user and the kernel. This file
|
|
descriptor is obtained by opening the device special file associated
|
|
with the scsi disk device (/dev/sdb) that was created during LUN
|
|
discovery. As per the location of the cxlflash driver within the
|
|
SCSI protocol stack, this open is actually not seen by the cxlflash
|
|
driver. Upon successful open, the user receives a file descriptor
|
|
(herein referred to as fd1) that should be used for issuing the
|
|
subsequent ioctls listed below.
|
|
|
|
The structure definitions for these IOCTLs are available in:
|
|
uapi/scsi/cxlflash_ioctl.h
|
|
|
|
DK_CXLFLASH_ATTACH
|
|
------------------
|
|
|
|
This ioctl obtains, initializes, and starts a context using the CXL
|
|
kernel services. These services specify a context id (u16) by which
|
|
to uniquely identify the context and its allocated resources. The
|
|
services additionally provide a second file descriptor (herein
|
|
referred to as fd2) that is used by the block library to initiate
|
|
memory mapped I/O (via mmap()) to the CXL flash device and poll for
|
|
completion events. This file descriptor is intentionally installed by
|
|
this driver and not the CXL kernel services to allow for intermediary
|
|
notification and access in the event of a non-user-initiated close(),
|
|
such as a killed process. This design point is described in further
|
|
detail in the description for the DK_CXLFLASH_DETACH ioctl.
|
|
|
|
There are a few important aspects regarding the "tokens" (context id
|
|
and fd2) that are provided back to the user:
|
|
|
|
- These tokens are only valid for the process under which they
|
|
were created. The child of a forked process cannot continue
|
|
to use the context id or file descriptor created by its parent
|
|
(see DK_CXLFLASH_VLUN_CLONE for further details).
|
|
|
|
- These tokens are only valid for the lifetime of the context and
|
|
the process under which they were created. Once either is
|
|
destroyed, the tokens are to be considered stale and subsequent
|
|
usage will result in errors.
|
|
|
|
- A valid adapter file descriptor (fd2 >= 0) is only returned on
|
|
the initial attach for a context. Subsequent attaches to an
|
|
existing context (DK_CXLFLASH_ATTACH_REUSE_CONTEXT flag present)
|
|
do not provide the adapter file descriptor as it was previously
|
|
made known to the application.
|
|
|
|
- When a context is no longer needed, the user shall detach from
|
|
the context via the DK_CXLFLASH_DETACH ioctl. When this ioctl
|
|
returns with a valid adapter file descriptor and the return flag
|
|
DK_CXLFLASH_APP_CLOSE_ADAP_FD is present, the application _must_
|
|
close the adapter file descriptor following a successful detach.
|
|
|
|
- When this ioctl returns with a valid fd2 and the return flag
|
|
DK_CXLFLASH_APP_CLOSE_ADAP_FD is present, the application _must_
|
|
close fd2 in the following circumstances:
|
|
|
|
+ Following a successful detach of the last user of the context
|
|
+ Following a successful recovery on the context's original fd2
|
|
+ In the child process of a fork(), following a clone ioctl,
|
|
on the fd2 associated with the source context
|
|
|
|
- At any time, a close on fd2 will invalidate the tokens. Applications
|
|
should exercise caution to only close fd2 when appropriate (outlined
|
|
in the previous bullet) to avoid premature loss of I/O.
|
|
|
|
DK_CXLFLASH_USER_DIRECT
|
|
-----------------------
|
|
This ioctl is responsible for transitioning the LUN to direct
|
|
(physical) mode access and configuring the AFU for direct access from
|
|
user space on a per-context basis. Additionally, the block size and
|
|
last logical block address (LBA) are returned to the user.
|
|
|
|
As mentioned previously, when operating in user space access mode,
|
|
LUNs may be accessed in whole or in part. Only one mode is allowed
|
|
at a time and if one mode is active (outstanding references exist),
|
|
requests to use the LUN in a different mode are denied.
|
|
|
|
The AFU is configured for direct access from user space by adding an
|
|
entry to the AFU's resource handle table. The index of the entry is
|
|
treated as a resource handle that is returned to the user. The user
|
|
is then able to use the handle to reference the LUN during I/O.
|
|
|
|
DK_CXLFLASH_USER_VIRTUAL
|
|
------------------------
|
|
This ioctl is responsible for transitioning the LUN to virtual mode
|
|
of access and configuring the AFU for virtual access from user space
|
|
on a per-context basis. Additionally, the block size and last logical
|
|
block address (LBA) are returned to the user.
|
|
|
|
As mentioned previously, when operating in user space access mode,
|
|
LUNs may be accessed in whole or in part. Only one mode is allowed
|
|
at a time and if one mode is active (outstanding references exist),
|
|
requests to use the LUN in a different mode are denied.
|
|
|
|
The AFU is configured for virtual access from user space by adding
|
|
an entry to the AFU's resource handle table. The index of the entry
|
|
is treated as a resource handle that is returned to the user. The
|
|
user is then able to use the handle to reference the LUN during I/O.
|
|
|
|
By default, the virtual LUN is created with a size of 0. The user
|
|
would need to use the DK_CXLFLASH_VLUN_RESIZE ioctl to adjust the grow
|
|
the virtual LUN to a desired size. To avoid having to perform this
|
|
resize for the initial creation of the virtual LUN, the user has the
|
|
option of specifying a size as part of the DK_CXLFLASH_USER_VIRTUAL
|
|
ioctl, such that when success is returned to the user, the
|
|
resource handle that is provided is already referencing provisioned
|
|
storage. This is reflected by the last LBA being a non-zero value.
|
|
|
|
When a LUN is accessible from more than one port, this ioctl will
|
|
return with the DK_CXLFLASH_ALL_PORTS_ACTIVE return flag set. This
|
|
provides the user with a hint that I/O can be retried in the event
|
|
of an I/O error as the LUN can be reached over multiple paths.
|
|
|
|
DK_CXLFLASH_VLUN_RESIZE
|
|
-----------------------
|
|
This ioctl is responsible for resizing a previously created virtual
|
|
LUN and will fail if invoked upon a LUN that is not in virtual
|
|
mode. Upon success, an updated last LBA is returned to the user
|
|
indicating the new size of the virtual LUN associated with the
|
|
resource handle.
|
|
|
|
The partitioning of virtual LUNs is jointly mediated by the cxlflash
|
|
driver and the AFU. An allocation table is kept for each LUN that is
|
|
operating in the virtual mode and used to program a LUN translation
|
|
table that the AFU references when provided with a resource handle.
|
|
|
|
DK_CXLFLASH_RELEASE
|
|
-------------------
|
|
This ioctl is responsible for releasing a previously obtained
|
|
reference to either a physical or virtual LUN. This can be
|
|
thought of as the inverse of the DK_CXLFLASH_USER_DIRECT or
|
|
DK_CXLFLASH_USER_VIRTUAL ioctls. Upon success, the resource handle
|
|
is no longer valid and the entry in the resource handle table is
|
|
made available to be used again.
|
|
|
|
As part of the release process for virtual LUNs, the virtual LUN
|
|
is first resized to 0 to clear out and free the translation tables
|
|
associated with the virtual LUN reference.
|
|
|
|
DK_CXLFLASH_DETACH
|
|
------------------
|
|
This ioctl is responsible for unregistering a context with the
|
|
cxlflash driver and release outstanding resources that were
|
|
not explicitly released via the DK_CXLFLASH_RELEASE ioctl. Upon
|
|
success, all "tokens" which had been provided to the user from the
|
|
DK_CXLFLASH_ATTACH onward are no longer valid.
|
|
|
|
When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
|
|
attach, the application _must_ close the fd2 associated with the context
|
|
following the detach of the final user of the context.
|
|
|
|
DK_CXLFLASH_VLUN_CLONE
|
|
----------------------
|
|
This ioctl is responsible for cloning a previously created
|
|
context to a more recently created context. It exists solely to
|
|
support maintaining user space access to storage after a process
|
|
forks. Upon success, the child process (which invoked the ioctl)
|
|
will have access to the same LUNs via the same resource handle(s)
|
|
as the parent, but under a different context.
|
|
|
|
Context sharing across processes is not supported with CXL and
|
|
therefore each fork must be met with establishing a new context
|
|
for the child process. This ioctl simplifies the state management
|
|
and playback required by a user in such a scenario. When a process
|
|
forks, child process can clone the parents context by first creating
|
|
a context (via DK_CXLFLASH_ATTACH) and then using this ioctl to
|
|
perform the clone from the parent to the child.
|
|
|
|
The clone itself is fairly simple. The resource handle and lun
|
|
translation tables are copied from the parent context to the child's
|
|
and then synced with the AFU.
|
|
|
|
When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
|
|
attach, the application _must_ close the fd2 associated with the source
|
|
context (still resident/accessible in the parent process) following the
|
|
clone. This is to avoid a stale entry in the file descriptor table of the
|
|
child process.
|
|
|
|
DK_CXLFLASH_VERIFY
|
|
------------------
|
|
This ioctl is used to detect various changes such as the capacity of
|
|
the disk changing, the number of LUNs visible changing, etc. In cases
|
|
where the changes affect the application (such as a LUN resize), the
|
|
cxlflash driver will report the changed state to the application.
|
|
|
|
The user calls in when they want to validate that a LUN hasn't been
|
|
changed in response to a check condition. As the user is operating out
|
|
of band from the kernel, they will see these types of events without
|
|
the kernel's knowledge. When encountered, the user's architected
|
|
behavior is to call in to this ioctl, indicating what they want to
|
|
verify and passing along any appropriate information. For now, only
|
|
verifying a LUN change (ie: size different) with sense data is
|
|
supported.
|
|
|
|
DK_CXLFLASH_RECOVER_AFU
|
|
-----------------------
|
|
This ioctl is used to drive recovery (if such an action is warranted)
|
|
of a specified user context. Any state associated with the user context
|
|
is re-established upon successful recovery.
|
|
|
|
User contexts are put into an error condition when the device needs to
|
|
be reset or is terminating. Users are notified of this error condition
|
|
by seeing all 0xF's on an MMIO read. Upon encountering this, the
|
|
architected behavior for a user is to call into this ioctl to recover
|
|
their context. A user may also call into this ioctl at any time to
|
|
check if the device is operating normally. If a failure is returned
|
|
from this ioctl, the user is expected to gracefully clean up their
|
|
context via release/detach ioctls. Until they do, the context they
|
|
hold is not relinquished. The user may also optionally exit the process
|
|
at which time the context/resources they held will be freed as part of
|
|
the release fop.
|
|
|
|
When the DK_CXLFLASH_APP_CLOSE_ADAP_FD flag was returned on a successful
|
|
attach, the application _must_ unmap and close the fd2 associated with the
|
|
original context following this ioctl returning success and indicating that
|
|
the context was recovered (DK_CXLFLASH_RECOVER_AFU_CONTEXT_RESET).
|
|
|
|
DK_CXLFLASH_MANAGE_LUN
|
|
----------------------
|
|
This ioctl is used to switch a LUN from a mode where it is available
|
|
for file-system access (legacy), to a mode where it is set aside for
|
|
exclusive user space access (superpipe). In case a LUN is visible
|
|
across multiple ports and adapters, this ioctl is used to uniquely
|
|
identify each LUN by its World Wide Node Name (WWNN).
|