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

["Chris Friesen" <cfriesen@nortel.com>]

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