FPU state management in L4Fiasco.OC program

Mai, Haohui haohui.mai at gmail.com
Mon Apr 9 00:48:07 CEST 2012


Hi,

To provide a more concrete idea for the discussion. Here is my
code skeleton:

static void server_loop(void)


{


        while (1) {


                ....
                handle_exception(&src, &tag);





                if (l4_msgtag_flags(tag) & L4_MSGTAG_TRANSFER_FPU)
                        l4_utcb_inherit_fpu_u(u, 1);



                tag = l4_ipc_reply_and_wait(u, tag, &src,
L4_IPC_SEND_TIMEOUT_0);




                l4_utcb_inherit_fpu_u(u, 0);
                ....
        }


}

static int handle_exception(l4_cap_idx_t *src_id, l4_msgtag_t *tag)
{
    l4_exc_regs_t exc;
    memcpy(&exc, l4_utcb_exc(), sizeof(exc));

    if (exc.trapno == 0x7) {
                printf("FPU exception\n");
                *tag = l4_msgtag(0, 0, 0, L4_MSGTAG_TRANSFER_FPU);
                return 0;
     }
     ...
}


Your comments are appreciated.

~Haohui

On Sun, Apr 8, 2012 at 5:22 PM, Mai, Haohui <haohui.mai at gmail.com> wrote:

> Hi,
>
> I'm writing a new OS kernel atop of L4Fiasco.OC. I'm having some
> difficulties to let my OS support user-level processes that uses
> floating-point instruction.
>
> So far, My OS closely follows L4Linux, it creates a new task for each
> user-level process, and marks as all threads of the task as alien thread to
> capture all page faults and exceptions.
>
> My OS works fines with page fault and syscalls. However, I'm confused with
> what I should do with floating-point exception (exception #7), which is
> issued to my kernel at the first time it tries to perform floating-point
> arithmetic.
>
> I tried to return to the user-level process with an empty message (
> l4_msgtag_t tag = l4_msgtag(0, 0, 0, 0) ), or an empty message with
> L4_MSGTAG_TRANSFER_FPU (l4_msgtag_t tag = l4_msgtag(0, 0, 0,
> L4_MSGTAG_TRANSFER_FPU) ), but I had no luck.
>
> The user-level process immediately calls "int $0x3" and stops.
>
> I read through L4Linux's code and it seems it's doing similar thing,
> except that it did initialize the FPU state before asking L4 to transfer
> the FPU state to user-level process.
>
> Am I missing something? What's the recommend way of doing it? Your
> comments are highly appreciated.
>
> ~Haohui
>
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