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    SubjectRe: [patch 05/11] syslets: core code
    On Wed, 14 Feb 2007, Davide Libenzi wrote:

    > On Wed, 14 Feb 2007, Ingo Molnar wrote:
    >
    > > yeah, that's another key thing. I do plan to provide a sys_upcall()
    > > syscall as well which calls a 5-parameter user-space function with a
    > > special stack. (it's like a lightweight signal/event handler, without
    > > any of the signal handler legacies and overhead - it's like a reverse
    > > system call - a "user call". Obviously pure userspace would never use
    > > sys_upcall(), unless as an act of sheer masochism.)
    >
    > That is exactly what I described as clets. Instead of having complex jump
    > and condition interpreters on the kernel (on top of new syscalls to
    > modify/increment userspace variables), you just code it in C and you pass
    > the clet pointer to the kernel.
    > The upcall will setup a frame, execute the clet (where jump/conditions and
    > userspace variable changes happen in machine code - gcc is pretty good in
    > taking care of that for us) on its return, come back through a
    > sys_async_return, and go back to userspace.

    So, for example, this is the setup code for the current API (and that's a
    really simple one - immagine going wacko with loops and userspace varaible
    changes):


    static struct req *alloc_req(void)
    {
    /*
    * Constants can be picked up by syslets via static variables:
    */
    static long O_RDONLY_var = O_RDONLY;
    static long FILE_BUF_SIZE_var = FILE_BUF_SIZE;

    struct req *req;

    if (freelist) {
    req = freelist;
    freelist = freelist->next_free;
    req->next_free = NULL;
    return req;
    }

    req = calloc(1, sizeof(struct req));

    /*
    * This is the first atom in the syslet, it opens the file:
    *
    * req->fd = open(req->filename, O_RDONLY);
    *
    * It is linked to the next read() atom.
    */
    req->filename_p = req->filename;
    init_atom(req, &req->open_file, __NR_sys_open,
    &req->filename_p, &O_RDONLY_var, NULL, NULL, NULL, NULL,
    &req->fd, SYSLET_STOP_ON_NEGATIVE, &req->read_file);

    /*
    * This second read() atom is linked back to itself, it skips to
    * the next one on stop:
    */
    req->file_buf_ptr = req->file_buf;
    init_atom(req, &req->read_file, __NR_sys_read,
    &req->fd, &req->file_buf_ptr, &FILE_BUF_SIZE_var,
    NULL, NULL, NULL, NULL,
    SYSLET_STOP_ON_NON_POSITIVE | SYSLET_SKIP_TO_NEXT_ON_STOP,
    &req->read_file);

    /*
    * This close() atom has NULL as next, this finishes the syslet:
    */
    init_atom(req, &req->close_file, __NR_sys_close,
    &req->fd, NULL, NULL, NULL, NULL, NULL, NULL, 0, NULL);

    return req;
    }


    Here's how your clet would look like:

    static long main_sync_loop(ctx *c)
    {
    int fd;
    char file_buf[FILE_BUF_SIZE+1];

    if ((fd = open(c->filename, O_RDONLY)) == -1)
    return -1;
    while (read(fd, file_buf, FILE_BUF_SIZE) > 0)
    ;
    close(fd);
    return 0;
    }


    Kinda easier to code isn't it? And the cost of the upcall to schedule the
    clet is widely amortized by the multple syscalls you're going to do inside
    your clet.




    - Davide


    -
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