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Date: Thu, 13 Dec 2012 17:49:49 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Stefani Seibold <stefani@seibold.net>, linux-kernel@vger.kernel.org,
        x86@kernel.org, tglx@linutronix.de, mingo@redhat.com,
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Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel
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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