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

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Message-ID: <50CA85BD.7070502@zytor.com>
Date:	Thu, 13 Dec 2012 17:49:49 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Stefani Seibold <stefani@...bold.net>,
	linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, ak@...ux.intel.com, aarcange@...hat.com,
	john.stultz@...aro.org
Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel

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

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