[PATCH v6 17/17] powerpc/vas: Document FTW API/usage
Sukadev Bhattiprolu
sukadev at linux.vnet.ibm.com
Wed Aug 9 09:07:02 AEST 2017
Document the usage of the VAS Fast thread-wakeup API.
Thanks for input/comments from Benjamin Herrenschmidt, Michael Neuling,
Michael Ellerman, Robert Blackmore, Ian Munsie, Haren Myneni, Paul Mackerras.
Cc:Ian Munsie <imunsie at au1.ibm.com>
Cc:Paul Mackerras <paulus at ozlabs.org>
Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
---
Documentation/powerpc/ftw-api.txt | 373 ++++++++++++++++++++++++++++++++++++++
1 file changed, 373 insertions(+)
create mode 100644 Documentation/powerpc/ftw-api.txt
diff --git a/Documentation/powerpc/ftw-api.txt b/Documentation/powerpc/ftw-api.txt
new file mode 100644
index 0000000..0b3f16f
--- /dev/null
+++ b/Documentation/powerpc/ftw-api.txt
@@ -0,0 +1,373 @@
+Virtual Accelerator Switchboard and Fast Thread-Wakeup API
+
+ Power9 processor supports a hardware subystem known as the Virtual
+ Accelerator Switchboard (VAS) which allows two entities in the Power9
+ system to efficiently exchange messages. Messages must be formatted as
+ Coprocessor Reqeust Blocks (CRB) and be submitted using the COPY/PASTE
+ instructions (new in Power9).
+
+ Usage of VAS depends on the entities exchanging the messages and
+ currently two usages have been identified.
+
+ First usage of VAS, referred to as VAS/NX involves a software thread
+ submitting data compression requests to a co-processor (hardware/nest
+ accelerator) aka NX engine. The API for this usage is described in the
+ VAS/NX API document.
+
+ Alternatively, VAS can be used by two software threads to efficiently
+ exchange messages. Initially, this mechanism is intended to wake up a
+ waiting thread quickly - i.e "fast thread wake-up (FTW)". This document
+ describes the user API for this VAS/FTW mechanism.
+
+ Application access to the FTW mechanism is provided through the NX-FTW
+ device node (/dev/crypto/nx-ftw) implemented by the VAS/FTW device
+ driver.
+
+ A software thread T1 that intends to wait for an event must first setup
+ a receive window, by opening the NX-FTW device and using the
+ VAS_RX_WIN_OPEN ioctl. Upon successful return from the VAS_RX_WIN_OPEN
+ ioctl, an rx_win_handle is returned.
+
+ A software thread T2 that intends to wake up T1 at some point, must first
+ set up a "send window" using the VAS_TX_WIN_OPEN ioctl and specify the
+ rx_win_handle obtained by T1. After a successful VAS_TX_WIN_OPEN ioctl the
+ send window of T2 is considered paired with the receive window of T1. The
+ thread T2 must then use mmap() to obtain a "paste address" for the send
+ window.
+
+ With this set up, thread T1 can wait for an event using the WAIT
+ instruction.
+
+ Thread T2 can wake up T1 by using the "COPY/PASTE" instructions and
+ submitting an empty/NULL CRB to the send window's paste address. The
+ wait/wake up process can be repeated as long as the threads have the
+ send/receive windows open.
+
+1. NX-FTW Device Node
+
+ There is one /dev/crypto/nx-ftw node in the system and it provides
+ access to the VAS/FTW functionality.
+
+ The only valid operations on the NX-FTW node are:
+
+ - open() the device for read and write.
+
+ - issue either VAS_RX_WIN_OPEN or VAS_TX_WIN_OPEN ioctls to set up
+ receive or send (only one of them per open).
+
+ - if the open is associated with send window (i.e VAS_TX_WIN_OPEN
+ ioctl was issued) mmap() the send window into the application's
+ virtual address space. (i.e get a 'paste_address' for the send
+ window).
+
+ - close the device node.
+
+ Other file operations on the NX-FTW node are undefined.
+
+ Note tHAT the COPY and PASTE operations go directly to the hardware
+ and not go through the NX-FTW device.
+
+ Although a system may have several instances of the VAS in the system
+ (typically, one per P9 chip) there is just one NX-FTW device node in
+ the system.
+
+ When the NX-FTW device node is opened, the kernel assigns a suitable
+ instance of VAS to the process. Kernel will make a best-effort attempt
+ to assign an optimal instance of VAS for the process. In the initial
+ release, the kernel does not support migrating the VAS instance if the
+ process migrates from a processor on one chip to a processor on another
+ chip.
+
+ Applications may chose a specific instance of the VAS using the 'vas_id'
+ field in the VAS_TX_WIN_OPEN and VAS_RX_WIN_OPEN ioctls as detailed below.
+
+2. Open NX-FTW node
+
+ The device should be opened for read and write. No special privileges
+ are needed to open the device. The device may be opened multiple times.
+
+ Each open() of the NX-FTW device may be associated with either a send
+ window or receive window but not both.
+
+ See open(2) system call man pages for other details such as return
+ values, error codes and restrictions.
+
+3. Setup Receive window (VAS_RX_WIN_OPEN ioctl)
+
+ A thread that expects to wait for events and be woken up using COPY/PASTE
+ must first set up a receive window by issuing the VAS_RX_WIN_OPEN ioctl.
+
+ #include <asm/vas.h>
+
+ struct vas_rx_win_open_attr rxattr;
+
+ rc = ioctl(fd, VAS_RX_WIN_OPEN, &rxattr);
+
+ The attributes of rxattr are as follows:
+
+ struct vas_rx_win_open_attr {
+ int16_t version;
+ int16_t vas_id;
+ int32_t rx_win_handle; /* output field */
+ int64_t reserved[8];
+ };
+
+ The version field identifies the version of the API and must currently
+ be set to 1.
+
+ The vas_id field identifies a specific instance of the VAS that the
+ application wishes to access. See section on VAS ID below.
+
+ The reserved field must be set to all zeroes.
+
+ Upon successful return from the ioctl, the rx_win_handle field contains
+ an identifier for the VAS window associated with this "sleeping" thread.
+
+ This rx_win_handle field is used to "pair" this receive window with a
+ send window and must be specified when opening the corresponding send
+ window (see struct vas_tx_win_open_attr below).
+
+ Return value:
+
+ The VAS_RX_WIN_OPEN ioctl returns 0 on success. On error, it returns -1
+ and sets the errno variable to indicate the error.
+
+ Error codes:
+
+ EINVAL version is invalid
+
+ EINVAL vas_id is invalid
+
+ EINVAL reserved field is not set to zeroes
+
+ EINVAL fd is already associated with a send window
+
+
+3. Set up a Send window (VAS_TX_WIN_OPEN ioctl)
+
+ An application thread that expects to wake up a waiting thread using
+ copy/paste, must first set up a send window that is paired with the
+ receive window of the waiting thread. This is accomplished using the
+ VAS_TX_WIN_OPEN ioctl.
+
+ #include <asm/vas.h>
+
+ struct vas_tx_win_open_attr txattr;
+
+ rc = ioctl(fd, VAS_TX_WIN_OPEN, &txattr);
+
+ The attributes 'txattr' for the VAS_TX_WIN_OPEN ioctl are defined as
+ follows:
+
+ struct vas_tx_win_open_attr {
+ int32_t version;
+ int16_t vas_id;
+ uint32_t rx_win_handle;
+
+ int64_t reserved1;
+
+ int64_t flags;
+ int64_t reserved2;
+
+ int32_t tc_mode;
+ int32_t rsvd_txbuf;
+ int64_t reserved3[6];
+ };
+
+ The version field must currently be set to 1.
+
+ The vas_id field identifies a specific instance of the VAS that the
+ application wishes to access. See section on VAS ID below.
+
+ The rx_win_handle field must be set to the rx_win_handle returned by
+ a prior successful call to VAS_RX_WIN_OPEN ioctl (see above). This
+ field is used to pair this send window with a receive window. The
+ process must have sufficient permissions to communicate with the
+ process owning the receive window identified by rx_win_handle.
+
+ The tc_mode and rsvd_txbuf fields are currently unused and must be
+ set to 0
+
+ The flags field specifies additional attributes to the window. The
+ only valid bit in the flag are for FTW windows is:
+
+ VAS_FLAGS_PIN_WINDOW if set, indicates the a window should be
+ pinned in cache. This flag is restricted
+ to privileged users. See Pinning windows
+ below.
+
+ All the other bits in the flags field must be set to 0.
+
+ The fields reserved1, reserved2 and reserved3 are for future extension
+ and must be set to 0.
+
+ Return value:
+
+ The VAS_TX_WIN_OPEN ioctl returns 0 on success. On error, it returns -1
+ and sets the errno variable to indicate the error.
+
+ Error conditions:
+
+ EINVAL version, vas_id or rx_win_handle fields are invalid
+
+ EINVAL fd does not refer to a valid VAS device.
+
+ EINVAL fd is already associated with a receive window
+
+ ENOSPC System has too many active windows (connections) open,
+
+ EINVAL For FTW windows, rsvd_txbuf is not 0.
+
+ EINVAL For FTW windows, tc_mode is not VAS_THRESH_DISABLED.
+
+ EPERM VAS_FLAGS_PIN_WINDOW is set in 'flags' field and process
+ is not privileged.
+
+ EPERM VAS_FLAGS_HIGH_PRI is set in 'flags' field and process
+ is not privileged.
+
+ EINVAL an invalid flag is set in the 'flags' field. (For FTW
+ windows, VAS_FLAGS_HIGH_PRI is also invalid).
+
+ EINVAL reserved fields are not set to 0.
+
+ See the ioctl(2) man page for more details, error codes and restrictions.
+
+4. mmap() NX-FTW device fd
+
+ The mmap() system call for a NX-FTW device fd returns a "paste address"
+ that the application can use to COPY/PASTE a CRB to the waiting thread.
+
+ paste_addr = mmap(NULL, size, prot, flags, fd, offset);
+
+ The mmap() operation is only valid on a file descriptor associated
+ with a send window.
+
+ Only restrictions on mmap for a NX-FTW device fd are:
+
+ - size parameter should be one page size
+
+ - offset parameter should be 0ULL.
+
+ Refer to mmap(2) man page for additional details/restrictions.
+
+ In addition to the error conditions listed on the mmap(2) man page,
+ mmap() can also fail with one of following error codes:
+
+ EINVAL fd is not associated with an open send window (i.e mmap()
+ does not follow a successful call to the VAS_TX_WIN_OPEN
+ ioctl).
+
+ EINVAL offset field is not 0ULL.
+
+
+5. VAS ID
+
+ A system may have several instances of VAS in the hardware, typically
+ one per POWER 9 chip. The choice of a specific instance of VAS can have
+ significant impact on the performance, specially if the application
+ migrates from one CPU to another. Applications can specify a vas_id
+ using the VAS_TX_WIN_OPEN and VAS_RX_WIN_OPEN ioctls and should be
+ prudent in choosing an instance of VAS.
+
+ The vas_id for each instance of VAS is listed as the device tree
+ property 'ibm,vas-id'. Determining the specific vas_id to use for
+ a specific application thread is beyond the scope of this API.
+
+ If the application has no preference, the vas_id field may be set to
+ -1 and the kernel will choose a suitable instance of the VAS engine.
+
+6. COPY/PASTE operations:
+
+ Applications should use the COPY and PASTE instructions defined in
+ the RFC to copy/paste the CRB. For VAS/FTW usage, the contents of
+ CRB if any, are ignored. CRB can be NULL.
+
+7. Interrupt completion and signal handling
+
+ No VAS-specific signals will be generated to the application threads
+ with the VAS/FTW usage.
+
+
+8. Example/Proposed usage of the VAS/FTW API
+
+ In the following example we use two threads that use the VAS/FTW API.
+ Thread T1 uses the WAIT instruction to wait for an event. Thread T2
+ uses copy/paste instructions to wake up T1.
+
+ Common interfaces:
+
+ static bool paste_done;
+ uint32_t rx_win_handle;
+
+ #define WAIT .long (0x7C00003C)
+
+ static inline int do_wait(void)
+ {
+ __asm__ __volatile(stringify_in_c(WAIT)";");
+ }
+
+ /*
+ * Check if paste_done is true
+ */
+ static bool is_paste_done(void)
+ {
+ return __sync_bool_compare_and_swap(&paste_done, 1, 0);
+
+ }
+
+ /*
+ * Set paste_done to true
+ */
+ static inline void set_paste_done(void)
+ {
+ __sync_bool_compare_and_swap(&paste_done, 0, 1);
+ }
+
+ Thread T1:
+
+ struct vas_rx_win_open_attr rxattr;
+
+ fd = open("/dev/crypto/nx-ftw", O_RDWR);
+
+ memset(&rxattr, 0, sizeof(rxattr));
+ rxattr.version = 1;
+
+ rc = ioctl(fd, VAS_RX_WIN_OPEN, &rxattr);
+
+ rx_win_handle = rxattr.rx_win_handle;
+
+ /* Tell T2 that Rx window is ready to be paired */
+ pthread_cond_signal(&rx_win_ready);
+
+ /* Rx set up done */
+
+ /* later, wait for an event to occur */
+
+ while(!is_paste_done())
+ do_wait();
+
+ Thread T2:
+
+ struct vas_tx_win_open_attr txattr;
+
+ fd = open("/dev/crypto/nx-ftw", O_RDWR);
+
+ /* Wait for Rx window to be set up first */
+ pthread_cond_wait(&rx_win_ready);
+
+ memset(&txattr, 0, sizeof(txattr));
+ txattr.version = 1;
+ txattr.rx_win_handle = rx_win_handle;
+
+ rc = ioctl(fd, VAS_TX_WIN_OPEN, &txattr);
+
+ prot = PROT_READ|PROT_WRITE;
+ paste_addr = mmap(NULL, 4096, prot, MAP_SHARED, fd, 0ULL);
+
+ /* Tx setup done */
+
+ /* later ... */
+
+ set_paste_done(); /* ... event occured */
+ write_null_crb(paste_addr); /* wake up T1 */
--
2.7.4
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