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.
Yes indeed. Any peculiarities that we should be watching out for?
Previous mailing list threads would be great.
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.
I haven't done mips in years and was a bit surprised that the
instruction set didn't provide for ways to read high and low words of a
64-bit value the way that ppc32 does.
unsigned long long gethrtime(void)
{
unsigned long 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.
I was a bit worried looking at the mips32 architecture manuals...didn't
realize that you could just flip to 64-bit mode like that.
Thanks for all the help.
Chris
