linux-kernel - Re: [PATCH 5/5] arm64: add Crypto Extensions based synchronous core AES cipher

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Date:	Mon, 3 Feb 2014 16:17:50 +0000
From:	Will Deacon <will.deacon@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 5/5] arm64: add Crypto Extensions based synchronous core
 AES cipher

On Thu, Jan 30, 2014 at 07:20:38PM +0000, Ard Biesheuvel wrote:
> On 30 January 2014 19:56, Will Deacon <will.deacon@....com> wrote:
> > On Wed, Jan 29, 2014 at 04:50:46PM +0000, Ard Biesheuvel wrote:
> >> +static void aes_cipher_encrypt(struct crypto_tfm *tfm, u8 dst[], u8 const src[])
> >> +{
> >> +     struct crypto_aes_ctx *ctx = crypto_tfm_ctx(tfm);
> >> +     u32 rounds = 6 + ctx->key_length / 4;
> >
> > Can you document these constants please?
> >
> 
> Sure.

Thanks.

> >> +
> >> +     kernel_neon_begin();
> >> +
> >> +     __asm__("       ld1             {v0.16b}, [%[in]]               ;"
> >> +             "       ld1             {v1.16b}, [%[key]], #16         ;"
> >> +             "0:     aese            v0.16b, v1.16b                  ;"
> >> +             "       subs            %[rounds], %[rounds], #1        ;"
> >> +             "       ld1             {v1.16b}, [%[key]], #16         ;"
> >> +             "       beq             1f                              ;"
> >> +             "       aesmc           v0.16b, v0.16b                  ;"
> >> +             "       b               0b                              ;"
> >> +             "1:     eor             v0.16b, v0.16b, v1.16b          ;"
> >> +             "       st1             {v0.16b}, [%[out]]              ;"
> >> +     : :
> >> +             [out]           "r"(dst),
> >> +             [in]            "r"(src),
> >> +             [rounds]        "r"(rounds),
> >> +             [key]           "r"(ctx->key_enc)
> >> +     :                       "cc");
> >
> > You probably need a memory output to stop this being re-ordered by the
> > compiler. Can GCC not generate the addressing modes you need directly,
> > allowing you to avoid moving everything into registers?
> >
> 
> Would a memory clobber work as well?

It would, but it could lead to suboptimal code generation by GCC (although
neon_{begin,end} may well stop GCC in its tracks anyway, so worth looking at
the disassembly).

> Re addressing modes: I would prefer to explicitly use v0 and v1, I
> have another patch pending that allows partial saves/restores of the
> NEON register file when called from interrupt context. I suppose I
> could use 'register asm("v0")' or something like that, but that won't
> make it any prettier.

It's not the use of v0/v1 that I was objecting to. I was hoping that we
could describe [in] and [out] as memory operands, so that GCC could
potentially reduce register usage for base + offset style addressing modes.
Unfortunately, I don't think we have such a constraint for AArch64 :(

If the disassembly looks ok, the "memory" clobber is probably our best bet.

Will
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