linux-kernel - Re: [PATCH 07/11] kexec: Create a relocatable object called purgatory

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Date:	Tue, 25 Feb 2014 08:55:42 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	ebiederm@...ssion.com, mjg59@...f.ucam.org, greg@...ah.com,
	jkosina@...e.cz
Subject: Re: [PATCH 07/11] kexec: Create a relocatable object called purgatory

On 02/25/2014 08:43 AM, Vivek Goyal wrote:
> 
> W.r.t sharing the code with arch/x86/boot/, I am not sure how to do it.
> 

Pretty much we have been doing #includes (a bit sad, I know)... there
are already a lot of them between arch/x86/boot,
arch/x86/boot/compressed, and arch/x86/realmode.  In that sense
collecting these "limited environments" together and have these kinds of
stuff together in one place seems like a good idea.

Does purgatory move large amounts of data around?  If so, we probably
*do* want to use rep movsl, but otherwise you're definitely right, using
C code makes more sense.

> I see two implementations of memcpy() under arch/x86/boot.
> 
> One is in copy.S. This is assembly code and looks like is supposed to
> run in 16bit mode. (code16).
> 
> Other one is in compressed/misc.c and there are two definitions, one
> for 32bit and one fore 64bit.

They are basically the 16-, 32-, and 64-bit variants of the same code.

> I am not sure why there is a need to write memcpy() in assembly when
> C will do just fine for my case. I don't have to write two versions of
> memcpy() and use it both for 32bit and 64bit.

The point would be to use the ones we already have.

> So I can just make all the purgatory code share same version of memcpy(),
> memcmp() and memset(), is that fine. I have taken implementations of
> these functions from lib/string.c

It depends on if you care about performance.  For memcpy() and memset()
in particular, the CPU has internally optimized versions of these that
beats C at least on any newer silicon.

	-hpa

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