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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