Re: [patch 05/11] syslets: core code

[Davide Libenzi]

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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