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From: Alberto Garcia <berto@igalia.com>
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Date: Tue, 09 Oct 2018 17:14:51 +0200
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Subject: Re: [Qemu-devel] [PATCH 3/6] crypto: introduce a xts_uint128 data
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To: =?utf-8?Q?Daniel_P=2E_Berrang=C3=A9?= <berrange@redhat.com>
Cc: qemu-devel@nongnu.org

On Tue 09 Oct 2018 04:58:39 PM CEST, Daniel P. Berrang=C3=A9 wrote:
>> > @@ -85,7 +90,7 @@ void xts_decrypt(const void *datactx,
>> >                   uint8_t *dst,
>> >                   const uint8_t *src)
>> >  {
>> > -    uint8_t PP[XTS_BLOCK_SIZE], CC[XTS_BLOCK_SIZE], T[XTS_BLOCK_SIZE];
>> > +    xts_uint128 PP, CC, T;
>> >      unsigned long i, m, mo, lim;
>>=20
>>    [...]
>>=20
>> >          /* Pm =3D first length % XTS_BLOCK_SIZE bytes of PP */
>> >          for (i =3D 0; i < mo; i++) {
>> > -            CC[i] =3D src[XTS_BLOCK_SIZE + i];
>> > -            dst[XTS_BLOCK_SIZE + i] =3D PP[i];
>> > +            ((uint8_t *)&CC)[i] =3D src[XTS_BLOCK_SIZE + i];
>> > +            dst[XTS_BLOCK_SIZE + i] =3D ((uint8_t *)&PP)[i];
>> >          }
>>=20
>> On second thoughts, these casts are a bit cumbersome. I wonder if it
>> isn't better to keep the array a uint8_t[] and only treat it as
>> xts_uint128 in the places where you actually do 64-bit operations
>> (xts_uint128_xor, xts_mult_x).
>
> I had done that originally, but it just shifts ugly casts from one
> place to another place in the code.

Does it really? There's a dozen casts to uint8_t * in different
places. If you use uint_8[] you would only need something like this:

static void xts_mult_x(uint8_t *I8)
{
    xts_uint128 *I =3D (xts_uint128 *) I8;
    /* ... the rest of the function remains the same ... */
}

And something similar in xts_uint128_xor(), which could be an inline
function instead of a macro.

Berto