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

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Date:	Wed, 5 May 2010 11:59:24 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Arnd Bergmann <arnd@...db.de>, 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 Tue, 04 May 2010 19:08:23 +0300
Avi Kivity <avi@...hat.com> wrote:

> On 05/04/2010 06:03 PM, Arnd Bergmann wrote:
> > On Tuesday 04 May 2010, Takuya Yoshikawa wrote:
...
> >> 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.
> >    
> 
> That's why the bitmaps are defined as little endian u64 aligned, even on 
> big endian 32-bit systems.  Little endian bitmaps are wordsize agnostic, 
> and u64 alignment ensures we can use long-sized bitops on mixed size 
> systems.

There was a suggestion to propose set_le_bit_user() kind of macros.
But what I thought was these have a constraint you two explained and seemed to be
a little bit specific to some area, like KVM.

So I decided to propose just the offset calculation macro.

  Thanks, Takuya
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