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SubjectRe: [PATCH v6 17/17] powerpc/vas: Document FTW API/usage
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On Tue, 2017-08-08 at 16:07 -0700, Sukadev Bhattiprolu wrote:
> 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@au1.ibm.com>
> Cc:Paul Mackerras <paulus@ozlabs.org>
> Signed-off-by: Sukadev Bhattiprolu <sukadev@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.

crypto?

> +
> +    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.

I realise there is a window here as part of the hardware implementation, but the
users don't care about the window on the receive side. It's hidden from them.
It's just an rx handle IMHO.

The sender certainly has a window that users care about since they have to mmap
it.

> +
> +    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.

I don't understand this statement

> +
> +    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.

How is it "optimal"?

> +    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);

So we talked about this offline before.... the fd here should not be from the
/dev device but should be the fd from rx_win_open ioctl.

As you have it here you pass the handle in as a parameter of ioctl. This means
all the permissions checks have to be done by you as to if these two windows can
be linked. If you use the fd from before, you can assume if the receiver has
given this fd to the sender, it has the right permissions.

I have some pseudo code at the end shows this.

> +    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.

Can this be different to the rx?

> +    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.

As above, this should be part of the FD otherwise users could specify anything
here and paste to anyone.

> +    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.

I would lean towards having 1 device per vas/chip but I'll defer to mpe and benh
on the best option here.

you planning a libftw to do this?

> +
> +    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.

+1

> +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.

+1

> +
> +
> +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.

So here's how pseudo code for my idea would look with pthreads.  

I've also added some memory barriers. The ISA suggests that copy/paste has no
ordering associated with it, so you are going to need them I think. I'm not sure
of the flavour though.

---
bool done = false;
int rxfd;

static void reciever(void)
{
do {
                asm("wait");
smp_mb(); /* needed for wait -> memory  */
} while (!done); /* check for spurious wakeup */
/* woken up! */
}

static void sender(void)
{
void *paste_addr;

/* mmap the rx file descriptor */
paste_addr = mmap(NULL, getpagesize(), prot, MAP_SHARED, rxfd, 0);

done = true;
smp_mb(); /* needed for memory -> paste */
        write_crb(paste_addr);
}

int main()
{
pthread_t thread;
int devfd;

        devfd = open("/dev/vas-ftw", O_RDWR);

/* create a new rx file descriptor associated with this LPID/PID/TID */
        rxfd = ioctl(devfd, VAS_RX_CREATE);

pthread_create(&thread, NULL, sender, NULL);

/* Reciever must *not* be a new thread since VAS_RX_CREATE
   ioctl is associated with this LPID/PID/TID 
*/
reciever();
}

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