From: Eric Biggers <ebiggers@kernel.org>
To: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: herbert@gondor.apana.org.au, davem@davemloft.net,
linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
kernel-janitors@vger.kernel.org
Subject: Re: [RFC PATCH] crypto: arc4: Implement a version optimized for memory usage
Date: Tue, 4 May 2021 12:36:21 -0700 [thread overview]
Message-ID: <YJGiNcorcgAcmAnb@gmail.com> (raw)
In-Reply-To: <d523902e-744c-1291-aee8-9be734f2a3ce@wanadoo.fr>
On Tue, May 04, 2021 at 07:59:38PM +0200, Christophe JAILLET wrote:
> Le 04/05/2021 à 18:57, Eric Biggers a écrit :
> > On Sun, May 02, 2021 at 09:29:46PM +0200, Christophe JAILLET wrote:
> > > +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> > > +#define S_type u8
> > > +#else
> > > +#define S_type u32
> > > +#endif
> > > +
> > > struct arc4_ctx {
> > > - u32 S[256];
> > > + S_type S[256];
> > > u32 x, y;
> > > };
> >
> > Is it actually useful to keep both versions? It seems we could just use the u8
> > version everywhere. Note that there aren't actually any unaligned memory
> > accesses, so choosing the version conditionally on
> > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS seems odd. What are you trying to
> > determine by checking that?
>
> Hi, this is a bad interpretation from me.
>
> I thought that S[1] would likely use an odd address and would trigger an
> unaligned access. But as we would read only 1 byte, this is not the case.
>
> Looking at [1], we have : "At this point, it should be clear that accessing
> a single byte (u8 or char) will never cause an unaligned access, because all
> memory addresses are evenly divisible by one."
>
>
> I wanted to avoid potential performance cost related to using char (i.e u8)
> instead of int (i.e. u32).
> On some architecture this could require some shift or masking or whatever to
> "unpack" the values of S.
>
>
> [1]:
> https://www.kernel.org/doc/html/latest/core-api/unaligned-memory-access.html
>
> CJ
>
arc4 is an insecure cipher which is only supported for use in legacy protocols.
So we don't really need to worry about optimizing performance on every
architecture. If the byte-based version is *usually* faster as well as uses
less memory, we probably should just use it everywhere.
- Eric
next prev parent reply other threads:[~2021-05-04 19:36 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-02 19:29 [RFC PATCH] crypto: arc4: Implement a version optimized for memory usage Christophe JAILLET
2021-05-04 16:57 ` Eric Biggers
2021-05-04 17:59 ` Christophe JAILLET
2021-05-04 19:36 ` Eric Biggers [this message]
2021-05-05 10:20 ` David Laight
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