[PATCH 5/5] powerpc/ftw: Document FTW API/usage
Randy Dunlap
rdunlap at infradead.org
Thu Jan 25 11:48:03 AEDT 2018
On 01/16/2018 06:50 PM, Sukadev Bhattiprolu wrote:
> Document the usage of the VAS Fast thread-wakeup API and add an entry in
> MAINTAINERS file.
>
> Thanks for input/comments from Benjamin Herrenschmidt, Michael Neuling,
> Michael Ellerman, Robert Blackmore, Ian Munsie, Haren Myneni and Paul
> Mackerras.
>
> Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> ---
>
> Changelog[v2]
> - [Michael Neuling] Update API to use a single, VAS_FTW_SEUTP ioctl
> rather than two ioctls.
> - [Michael Neuling] Drop "nx" from name "nx-ftw".
>
> ---
> Documentation/powerpc/ftw-api.txt | 283 ++++++++++++++++++++++++++++++++++++++
> MAINTAINERS | 8 ++
> 2 files changed, 291 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..a107628
> --- /dev/null
> +++ b/Documentation/powerpc/ftw-api.txt
> @@ -0,0 +1,283 @@
> +Virtual Accelerator Switchboard and Fast Thread-Wakeup API
> +
...
> +
> + Application access to the FTW mechanism is provided through the FTW
> + device node (/dev/ftw) implemented by the FTW device driver.
> +
> + A multi-threaded software processes that intends to use the FTW
process
> + mechanism must first setup a channel (consisting of a pair of VAS
> + windows) for the waiting and waking threads to communicate. The
> + channel is set up by opening the FTW device and issuing the FTW_SETUP
> + ioctl. Upon successful return from the ioctl, the waiting side of
> + channel is complete and a thread can issue the "Wait" instruction
> + to wait for an event.
> +
> + After the successful return from the FTW_SETUP ioctl, the waking
> + thread must use mmap() system call on the same file descriptor and
> + obtain a virtual address known as the "paste address".
> +
> + Once the mmap() call succeeds the setup of "waking" side of the channel
> + is complete. To wake up a waiting thread, the waking thread should use
> + the "COPY" and "PASTE" instructions to write a zero-filled CRB to the
> + paste-address.
> +
> + The wait and wake up operations can be repeated as long as the paste
> + address and the FTW file descriptor are valid (i.e until munmap() of
> + the paste address or a close() of the FTW fd).
> +
> +1. FTW Device Node
> +
> + There is one /dev/ftw node in the system and it provides access to the
> + VAS/FTW functionality.
> +
> + The only valid operations (system calls) on the FTW node are:
> +
> + - open() the device for read and write.
> +
> + - issue the FTW_SETUP ioctl to set up a channel.
> +
> + - mmap() the file descriptor
> +
> + - close the device node.
> +
> + Other file operations on the FTW node are undefined.
> +
> + Note that the COPY and PASTE operations go directly to the hardware
> + and do not involve system calls or go through the FTW device.
> +
> + Although a system may have several instances of the VAS in the system
> + (typically, one per P9 chip) there is just one FTW device node in
> + the system.
> +
> + When the 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 - based on the CPU/
> + chip that the process is running on. In the initial release, the kernel
> + does not support migrating the VAS instance if the process migrates from
> + a CPU on one chip to a CPU on another chip.
> +
> + Applications may chose a specific instance of the VAS using the 'vas_id'
choose
> + field in the FTW_SETUP ioctl as detailed below.
> +
> +2. Open 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 FTW device is associated with one channel of
> + communication. There is a system-wide limit (currently 64K windows per
> + chip and since some are reserved for hardware, there are about 32K
> + channels per chip). If no more channels are available, the open() system
> + call will fail.
> +
> + See open(2) system call man pages for other details such as return
> + values, error codes and restrictions.
> +
> +3. Setup a communication channel (FTW_SETUP ioctl)
> +
> + A process that intends to use the Fast Thread-wakeup mechanism must
> + first setup a channel by issuing the FTW_SETUP ioctl.
> +
> + #include <misc/ftw.h>
> +
> + struct ftw_setup_attr ftwattr;
> +
> + rc = ioctl(fd, FTW_SETUP, &ftwattr);
> +
> + The attributes of ftwattr are as follows:
> +
> + struct ftw_setup_attr {
> + int16_t version;
> + int16_t vas_id;
> + uint32_t reserved;
> +
> + int64_t reserved1;
> + int64_t flags;
> + int64_t reserved2;
> + };
> +
> + 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 fields must all be set to zeroes.
> +
> + The flags field specifies additional attributes to the channel. The
> + only valid bit in the flags for Fast thread-wakeup usage are:
> +
> + FTW_FLAGS_PIN_WINDOW if set, indicates that the channel 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.
> +
> + Return value:
> +
> + The FTW_SETUP 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 fd does not refer to a valid VAS device.
> +
> + ENOSPC System has too many active channels (windows) open,
s/,/./ or s/,//
> +
> + EPERM FTW_FLAGS_PIN_WINDOW is set in 'flags' field and process
> + is not privileged.
> +
> + EINVAL reserved fields are not set to 0.
> +
> + See the ioctl(2) man page for more details, error codes and restrictions.
> +
> +4. mmap() FTW device fd
> +
> + The mmap() system call for a 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);
> +
> + Only restrictions on mmap for a 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 channel (window)
> + (i.e mmap() does not follow a successful call to the
> + FTW_SETUP ioctl).
> +
> + EINVAL offset field is not 0ULL.
> +
> +
> +5. VAS ID
...
> +
> +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, are ignored and can be zero, but CRB should point to a valid buffer
CRB are ignored buffer.
> +
> +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 sets up the channel and uses the WAIT instruction to wait for
> + an event. Thread T2 uses copy/paste instructions to wake up T1.
> + Note that the pthread_cond_wait() calls must be in a loop for spurious
> + wake ups, but are simplified here.
> +
> + Common interfaces:
> +
> + static bool paste_done;
> +
> + #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);
> + }
> +
> +
> + int fd = -1; // global, visible to both T1 and T2
> +
> + Thread T1:
> +
> + struct ftw_setup_attr ftwattr;
> +
> + fd = open("/dev/ftw", O_RDWR);
> +
> + memset(&rxattr, 0, sizeof(rxattr));
Is that supposed to be ftwattr (2x above)?
> + ftwattr.version = 1;
> + ftwattr.vas_id = -1;
> +
> + rc = ioctl(fd, FTW_SETUP, &ftwattr);
> +
> + /* Tell T2 that waiter side of channel is ready */
> + 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:
> +
> + /* Wait for waiter side of channel to be set up first */
> + pthread_cond_wait(&rx_win_ready);
> +
> + prot = PROT_READ|PROT_WRITE;
> + paste_addr = mmap(NULL, 4096, prot, MAP_SHARED, fd, 0ULL);
> +
> + /* Tx setup done */
> +
> + /* later ... */
> +
> + set_paste_done(); /* ... event occurred */
> + write_empty_crb(paste_addr); /* wake up T1 */
--
~Randy
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