Re: quick question on 64-bit values with 32-bit inline assembly

[Ralf Baechle <ralf@linux-mips.org>]

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