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

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: quick question on 64-bit values with 32-bit inline assembly
From: "Chris Friesen" <cfriesen@nortel.com>
Date: Tue, 22 Jan 2008 12:55:45 -0600
Cc: linux-mips@linux-mips.org
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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


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