[Top] [All Lists]

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

To: Chris Friesen <>
Subject: Re: quick question on 64-bit values with 32-bit inline assembly
From: Ralf Baechle <>
Date: Tue, 22 Jan 2008 17:57:34 +0000
In-reply-to: <>
Original-recipient: rfc822;
References: <>
User-agent: Mutt/1.5.17 (2007-11-01)
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.


<Prev in Thread] Current Thread [Next in Thread>