On Tue, 22 Jan 2008, Ralf Baechle wrote:
> On Mon, Jan 21, 2008 at 12:09:37PM -0600, Chris Friesen wrote:
> > We're running a 64-bit kernel and 32-bit userspace. We've got some code
> > that is trying to get a 64-bit timestamp in userspace.
> >
> > The following code seems to work fine in the kernel but in userspace it
> > appears to be swapping the two words in the result.
> >
> > gethrtime(void)
> > {
> > unsigned long long result;
> >
> > asm volatile ("rdhwr %0,$31" : "=r" (result));
>
> Ah, Cavium.
>
> > return result;
> > }
> >
> > Do I need to do something special because userspace is 32-bit? If so, can
> > someone point me to a reference?
>
> Ouch. You found a nasty special case. Normally 32-bit userspace should
> not use 64-bit values but since you're running a 64-bit kernel.
>
> unsigned long long gethrtime(void)
> {
> unsigned long result;
^^^^^^^^^^^^^
unsigned long long
> asm volatile(
> " .set mips64r2 \n"
> " rdhwr %M0, $31 \n"
> " sll %L0, %M0, 0 \n"
> " dsra %M0, 32 \n"
> " .set mips0 \n"
> : "=r" (result));
>
> return result;
> }
>
> Note this wouldn't possibly work on a 32-bit kernel because 32-bit kernels
> will corrupt the upper 32-bit of integer registers so you might lose the
> result value before you can stash it away. Also 32-bit kernels don't allow
> the execution of 64-bit instructions, not even on 64-bit processors.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
