linux-kernel - Re: [RFC PATCH] crypto: riscv: scalar accelerated GHASH

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Message-ID: <CAMj1kXH-u7hiKGQfgYHj_16V4ATN_aHmA_wkvMSyLh+E3+QaAA@mail.gmail.com>
Date: Thu, 17 Apr 2025 09:57:49 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Qingfang Deng <dqfext@...il.com>
Cc: Eric Biggers <ebiggers@...nel.org>, Herbert Xu <herbert@...dor.apana.org.au>, 
	"David S. Miller" <davem@...emloft.net>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Alexandre Ghiti <alex@...ti.fr>, linux-crypto@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Christoph Müllner <christoph.muellner@...ll.eu>, 
	Heiko Stuebner <heiko.stuebner@...ll.eu>, Qingfang Deng <qingfang.deng@...lower.com.cn>
Subject: Re: [RFC PATCH] crypto: riscv: scalar accelerated GHASH

On Thu, 17 Apr 2025 at 09:25, Qingfang Deng <dqfext@...il.com> wrote:
>
> Hi Ard,
>
> On Thu, Apr 17, 2025 at 2:58 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > (cc Eric)
> >
> > On Thu, 17 Apr 2025 at 08:49, Qingfang Deng <dqfext@...il.com> wrote:
> > >
> > > From: Qingfang Deng <qingfang.deng@...lower.com.cn>
> > >
> > > Add a scalar implementation of GHASH for RISC-V using the Zbc (carry-less
> > > multiplication) and Zbb (bit-manipulation) extensions. This implementation
> > > is adapted from OpenSSL but rewritten in plain C for clarity.
> > >
> > > Unlike the OpenSSL one that rely on bit-reflection of the data, this
> > > version uses a pre-computed (reflected and multiplied) key, inspired by
> > > the approach used in Intel's CLMUL driver, to avoid reflections during
> > > runtime.
> > >
> > > Signed-off-by: Qingfang Deng <qingfang.deng@...lower.com.cn>
> >
> > What is the use case for this? AIUI, the scalar AES instructions were
> > never implemented by anyone, so how do you expect this to be used in
> > practice?
>
> The use case _is_ AES-GCM, as you mentioned. Without this, computing
> GHASH can take a considerable amount of CPU time (monitored by perf).
>

I see. But do you have a particular configuration in mind? Does it
have scalar AES too? I looked into that a while ago but I was told
that nobody actually incorporates that. So what about these
extensions? Are they commonly implemented?

[0] https://web.git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=riscv-scalar-aes

> > ...
> > > +static __always_inline __uint128_t get_unaligned_be128(const u8 *p)
> > > +{
> > > +       __uint128_t val;
> > > +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> >
> > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS means that get_unaligned_xxx()
> > helpers are cheap. Casting a void* to an aligned type is still UB as
> > per the C standard.
>
> Technically an unaligned access is UB but this pattern is widely used
> in networking code.
>

Of course. But that is no reason to keep doing it.

> >
> > So better to drop the #ifdef entirely, and just use the
> > get_unaligned_be64() helpers for both cases.
>
> Currently those helpers won't generate rev8 instructions, even if
> HAVE_EFFICIENT_UNALIGNED_ACCESS and RISCV_ISA_ZBB is set, so I have to
> implement my own version of this to reduce the number of instructions,
> and to align with the original OpenSSL implementation.
>

So fix the helpers.