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;
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.
Ralf
