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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: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Tue, 22 Jan 2008 19:02:02 +0100 (CET)
Cc: Chris Friesen <cfriesen@nortel.com>, linux-mips@linux-mips.org
In-reply-to: <20080122175734.GA31013@linux-mips.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <4794DFE1.5040805@nortel.com> <20080122175734.GA31013@linux-mips.org>
Sender: linux-mips-bounce@linux-mips.org
On Tue, 22 Jan 2008, 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.
> 
> >    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;
        ^^^^^^^^^^^^^
        unsigned long long

>       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.

Gr{oetje,eeting}s,

                                                Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                                            -- Linus Torvalds

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