Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

From: H. Peter Anvin
Date: Thu Dec 13 2012 - 20:50:14 EST


On 12/13/2012 05:42 PM, Andy Lutomirski wrote:
>
> The 64-bit/x32 case is currently very simple and fast because it uses
> absolute addressing. Admittedly, pcrel references are free, so
> changing this wouldn't cost much. For native, it'll be slower, but
> maybe no one cares. I seem to care about this more than anyone else,
> and I don't use 32 bit code. :)
>

pcrel is actually cheaper than absolute addressing in 64-bit mode.

> The benefit of switching is that the vdso code could be the same in
> all three cases. (Actually, it's even better than that. All of the
> VVAR magic could be the same in the vdso and the kernel -- the kernel
> linker script would just have to have an appropriate symbol to see the
> appropriate mapping.)
>
>
> This:
>
> __attribute__((visibility("hidden"))) int foo;
>
> int get_foo(void)
> {
> return foo;
> }
>
> generates a rip-relative access on 64 bits and GOTOFF on 32 bits.
>
> The only reason I didn't use a real symbol in the first place is
> because I couldn't figure out how to get gcc to emit an absolute
> relocation in pic code.

Well, then, we wouldn't need to do that... this is starting to sound
like a significant win.

-hpa

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/