Re: [RFC][PATCH resend 8/12] asm-generic: bitops: introduce le bit offset macro
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On 05/04/2010 06:03 PM, Arnd Bergmann wrote:
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.
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.
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