linux-kernel - Re: Word-at-a-time dcache name accesses (was Re: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?)

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Date:	Fri, 2 Mar 2012 16:24:11 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	david@...g.hm
Cc:	"Ted Ts'o" <tytso@....edu>, Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Word-at-a-time dcache name accesses (was Re: .. anybody know of
 any filesystems that depend on the exact VFS 'namehash' implementation?)

On Fri, Mar 2, 2012 at 4:17 PM,  <david@...g.hm> wrote:
>
> or did some CPUs have efficient char access, but inefficient unaligned word
> access?

Tons of CPU's have efficient char accesses but horrible unaligned word
accesses. Some are even outright buggy (ie at least some ARM cores)
and load crap. Others take a fault.

They just aren't x86, because x86 has traditionally had code with a
fair amount of unaligned loads and stores (and not just for historical
reasons either: even modern code replaces constant-sized memcpy() etc
with direct loads and stores)

For some other architectures, we could just use "get_unaligned()",
which fixes things up for them. I could have made that explicit, even
if it doesn't matter on x86.

So the bigger portability problem to some degree is the fact that it
is limited to little-endian, so even if you have a CPU with good
unaligned accesses (some POWER chips do ok, for example, although not
all), you'd have to also do something with the mask generation (which
currently uses the "(x-1)&~x" trick that means that it generates the
mask of the *low bits* - and then assumes that "low bits" means "first
bytes" - ie little endian).

                       Linus
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