linux-kernel - Re: [RFC][PATCH resend 8/12] asm-generic: bitops: introduce le bit offset macro

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Date:	Tue, 4 May 2010 17:03:00 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
Cc:	avi@...hat.com, mtosatti@...hat.com, agraf@...e.de,
	yoshikawa.takuya@....ntt.co.jp, fernando@....ntt.co.jp,
	kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
	kvm-ia64@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, benh@...nel.crashing.org,
	paulus@...ba.org, linuxppc-dev@...abs.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH resend 8/12] asm-generic: bitops: introduce le bit offset macro

On Tuesday 04 May 2010, Takuya Yoshikawa wrote:
> 
> Although we can use *_le_bit() helpers to treat bitmaps le arranged,
> having le bit offset calculation as a seperate macro gives us more freedom.
> 
> For example, KVM has le arranged dirty bitmaps for VGA, live-migration
> and they are used in user space too. To avoid bitmap copies between kernel
> and user space, we want to update the bitmaps in user space directly.
> To achive this, le bit offset with *_user() functions help us a lot.
> 
> So let us use the le bit offset calculation part by defining it as a new
> macro: generic_le_bit_offset() .

Does this work correctly if your user space is 32 bits (i.e. unsigned long
is different size in user space and kernel) in both big- and little-endian
systems?

I'm not sure about all the details, but I think you cannot in general share
bitmaps between user space and kernel because of this.

	Arnd
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