linux-kernel - Re: [PATCH 4/4] Add 32 bit VDSO time support for 64 bit kernel

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Date:	Sat, 01 Feb 2014 12:29:01 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	stefani@...bold.net, gregkh@...uxfoundation.org,
	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, luto@...capital.net, xemul@...allels.com,
	gorcunov@...nvz.org, andriy.shevchenko@...ux.intel.com
CC:	Martin.Runge@...de-schwarz.com, Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH 4/4] Add 32 bit VDSO time support for 64 bit kernel

On 02/01/2014 07:32 AM, stefani@...bold.net wrote:
> 
> This kind of type hacking could be prevent in the future by doing a call to the
> 64 bit code by the following sequence:
> 
> - Compile the arch/x86/vdso/vclock_gettime.c as 64 bit, but only generate
>   the assembler output.
> - Next compile a 32 bit object by including the 64 bit vclock_gettime.s
>   prefixed with .code64
> - At least we need a trampolin code which invokes the 64 bit code and do
>   the API conversation (64 bit longs to 32 bit longs), like the
>   followig snipped:
> 

Honestly, I think the overhead of the mode switch would ruin a lot of
the advantages with the vdso.

	-hpa


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