Re: [PATCH v3 7/8] crypto: arm64/polyval: Add PMULL accelerated implementation of POLYVAL

[Nathan Huckleberry <nhuck@google.com>]

On Wed, Mar 23, 2022 at 8:37 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Tue, Mar 15, 2022 at 11:00:34PM +0000, Nathan Huckleberry wrote:
> > Add hardware accelerated version of POLYVAL for ARM64 CPUs with
> > Crypto Extension support.
>
> Nit: It's "Crypto Extensions", not "Crypto Extension".
>
> > +config CRYPTO_POLYVAL_ARM64_CE
> > +     tristate "POLYVAL using ARMv8 Crypto Extensions (for HCTR2)"
> > +     depends on KERNEL_MODE_NEON
> > +     select CRYPTO_CRYPTD
> > +     select CRYPTO_HASH
> > +     select CRYPTO_POLYVAL
>
> CRYPTO_POLYVAL selects CRYPTO_HASH already, so there's no need to select it
> here.
>
> > +/*
> > + * Perform polynomial evaluation as specified by POLYVAL.  This computes:
> > + *   h^n * accumulator + h^n * m_0 + ... + h^1 * m_{n-1}
> > + * where n=nblocks, h is the hash key, and m_i are the message blocks.
> > + *
> > + * x0 - pointer to message blocks
> > + * x1 - pointer to precomputed key powers h^8 ... h^1
> > + * x2 - number of blocks to hash
> > + * x3 - pointer to accumulator
> > + *
> > + * void pmull_polyval_update(const u8 *in, const struct polyval_ctx *ctx,
> > + *                        size_t nblocks, u8 *accumulator);
> > + */
> > +SYM_FUNC_START(pmull_polyval_update)
> > +     adr             TMP, .Lgstar
> > +     ld1             {GSTAR.2d}, [TMP]
> > +     ld1             {SUM.16b}, [x3]
> > +     ands            PARTIAL_LEFT, BLOCKS_LEFT, #7
> > +     beq             .LskipPartial
> > +     partial_stride
> > +.LskipPartial:
> > +     subs            BLOCKS_LEFT, BLOCKS_LEFT, #NUM_PRECOMPUTE_POWERS
> > +     blt             .LstrideLoopExit
> > +     ld1             {KEY8.16b, KEY7.16b, KEY6.16b, KEY5.16b}, [x1], #64
> > +     ld1             {KEY4.16b, KEY3.16b, KEY2.16b, KEY1.16b}, [x1], #64
> > +     full_stride 0
> > +     subs            BLOCKS_LEFT, BLOCKS_LEFT, #NUM_PRECOMPUTE_POWERS
> > +     blt             .LstrideLoopExitReduce
> > +.LstrideLoop:
> > +     full_stride 1
> > +     subs            BLOCKS_LEFT, BLOCKS_LEFT, #NUM_PRECOMPUTE_POWERS
> > +     bge             .LstrideLoop
> > +.LstrideLoopExitReduce:
> > +     montgomery_reduction
> > +     mov             SUM.16b, PH.16b
> > +.LstrideLoopExit:
> > +     st1             {SUM.16b}, [x3]
> > +     ret
> > +SYM_FUNC_END(pmull_polyval_update)
>
> Is there a reason why partial_stride is done first in the arm64 implementation,
> but last in the x86 implementation?  It would be nice if the implementations
> worked the same way.  Probably last would be better?  What is the advantage of
> doing it first?

It was so I could return early without loading keys into registers,
since I only need them if there's
a full stride. I was able to rewrite it in the same way that the x86
implementation works.
>
> Besides that, many of the comments I made on the x86 implementation apply to the
> arm64 implementation too.
>
> - Eric

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