netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 179 insertions, 0 deletions

diff --git a/Documentation/userspace-api/accelerators/ocxl.rst b/Documentation/userspace-api/accelerators/ocxl.rst
new file mode 100644
index 000000000..4e213af70
--- /dev/null
+++ b/Documentation/userspace-api/accelerators/ocxl.rst
@@ -0,0 +1,179 @@
+========================================================
+OpenCAPI (Open Coherent Accelerator Processor Interface)
+========================================================
+
+OpenCAPI is an interface between processors and accelerators. It aims
+at being low-latency and high-bandwidth.
+
+The specification was developed by the OpenCAPI Consortium, and is now
+available from the `Compute Express Link Consortium
+<https://computeexpresslink.org/resource/opencapi-specification-archive/>`_.
+
+It allows an accelerator (which could be an FPGA, ASICs, ...) to access
+the host memory coherently, using virtual addresses. An OpenCAPI
+device can also host its own memory, that can be accessed from the
+host.
+
+OpenCAPI is known in linux as 'ocxl', as the open, processor-agnostic
+evolution of 'cxl' (the driver for the IBM CAPI interface for
+powerpc), which was named that way to avoid confusion with the ISDN
+CAPI subsystem.
+
+
+High-level view
+===============
+
+OpenCAPI defines a Data Link Layer (DL) and Transaction Layer (TL), to
+be implemented on top of a physical link. Any processor or device
+implementing the DL and TL can start sharing memory.
+
+::
+
+  +-----------+                         +-------------+
+  |           |                         |             |
+  |           |                         | Accelerated |
+  | Processor |                         |  Function   |
+  |           |  +--------+             |    Unit     |  +--------+
+  |           |--| Memory |             |    (AFU)    |--| Memory |
+  |           |  +--------+             |             |  +--------+
+  +-----------+                         +-------------+
+       |                                       |
+  +-----------+                         +-------------+
+  |    TL     |                         |    TLX      |
+  +-----------+                         +-------------+
+       |                                       |
+  +-----------+                         +-------------+
+  |    DL     |                         |    DLX      |
+  +-----------+                         +-------------+
+       |                                       |
+       |                   PHY                 |
+       +---------------------------------------+
+
+
+
+Device discovery
+================
+
+OpenCAPI relies on a PCI-like configuration space, implemented on the
+device. So the host can discover AFUs by querying the config space.
+
+OpenCAPI devices in Linux are treated like PCI devices (with a few
+caveats). The firmware is expected to abstract the hardware as if it
+was a PCI link. A lot of the existing PCI infrastructure is reused:
+devices are scanned and BARs are assigned during the standard PCI
+enumeration. Commands like 'lspci' can therefore be used to see what
+devices are available.
+
+The configuration space defines the AFU(s) that can be found on the
+physical adapter, such as its name, how many memory contexts it can
+work with, the size of its MMIO areas, ...
+
+
+
+MMIO
+====
+
+OpenCAPI defines two MMIO areas for each AFU:
+
+* the global MMIO area, with registers pertinent to the whole AFU.
+* a per-process MMIO area, which has a fixed size for each context.
+
+
+
+AFU interrupts
+==============
+
+OpenCAPI includes the possibility for an AFU to send an interrupt to a
+host process. It is done through a 'intrp_req' defined in the
+Transaction Layer, specifying a 64-bit object handle which defines the
+interrupt.
+
+The driver allows a process to allocate an interrupt and obtain its
+64-bit object handle, that can be passed to the AFU.
+
+
+
+char devices
+============
+
+The driver creates one char device per AFU found on the physical
+device. A physical device may have multiple functions and each
+function can have multiple AFUs. At the time of this writing though,
+it has only been tested with devices exporting only one AFU.
+
+Char devices can be found in /dev/ocxl/ and are named as:
+/dev/ocxl/<AFU name>.<location>.<index>
+
+where <AFU name> is a max 20-character long name, as found in the
+config space of the AFU.
+<location> is added by the driver and can help distinguish devices
+when a system has more than one instance of the same OpenCAPI device.
+<index> is also to help distinguish AFUs in the unlikely case where a
+device carries multiple copies of the same AFU.
+
+
+
+Sysfs class
+===========
+
+An ocxl class is added for the devices representing the AFUs. See
+/sys/class/ocxl. The layout is described in
+Documentation/ABI/testing/sysfs-class-ocxl
+
+
+
+User API
+========
+
+open
+----
+
+Based on the AFU definition found in the config space, an AFU may
+support working with more than one memory context, in which case the
+associated char device may be opened multiple times by different
+processes.
+
+
+ioctl
+-----
+
+OCXL_IOCTL_ATTACH:
+
+  Attach the memory context of the calling process to the AFU so that
+  the AFU can access its memory.
+
+OCXL_IOCTL_IRQ_ALLOC:
+
+  Allocate an AFU interrupt and return an identifier.
+
+OCXL_IOCTL_IRQ_FREE:
+
+  Free a previously allocated AFU interrupt.
+
+OCXL_IOCTL_IRQ_SET_FD:
+
+  Associate an event fd to an AFU interrupt so that the user process
+  can be notified when the AFU sends an interrupt.
+
+OCXL_IOCTL_GET_METADATA:
+
+  Obtains configuration information from the card, such at the size of
+  MMIO areas, the AFU version, and the PASID for the current context.
+
+OCXL_IOCTL_ENABLE_P9_WAIT:
+
+  Allows the AFU to wake a userspace thread executing 'wait'. Returns
+  information to userspace to allow it to configure the AFU. Note that
+  this is only available on POWER9.
+
+OCXL_IOCTL_GET_FEATURES:
+
+  Reports on which CPU features that affect OpenCAPI are usable from
+  userspace.
+
+
+mmap
+----
+
+A process can mmap the per-process MMIO area for interactions with the
+AFU.