Re: [Qemu-devel] [PATCH v2 5/8] crypto: convert xts_mult_x to use xts_uint128 type

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Oct 16, 2018 at 03:35:01PM +0200, Alberto Garcia wrote:
> On Tue 16 Oct 2018 12:09:15 PM CEST, Daniel P. Berrangé wrote:
> > Using 64-bit arithmetic increases the performance for xts-aes-128
> > when built with gcrypt:
> >
> >   Encrypt: 355 MB/s -> 545 MB/s
> >   Decrypt: 362 MB/s -> 568 MB/s
> >
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> 
> This patch is also fine, but I have a couple of minor comments:
> 
> > +static void xts_mult_x(xts_uint128 *I)
> > +{
> > +    uint64_t tt;
> > +
> > +    xts_uint128_cpu_to_les(I);
> > +
> > +    tt = I->u[0] >> 63;
> > +    I->u[0] = I->u[0] << 1;
> 
> Perhaps I->u[0] <<= 1 , for clarity and consistency with the following
> line (I->u[0] ^= 0x87) ? But I don't mind if you prefer to keep it as is
> now.

In fact I could do the following:

@@ -59,12 +59,13 @@ static void xts_mult_x(xts_uint128 *I)
     xts_uint128_cpu_to_les(I);
 
     tt = I->u[0] >> 63;
-    I->u[0] = I->u[0] << 1;
+    I->u[0] <<= 1;
 
     if (I->u[1] >> 63) {
         I->u[0] ^= 0x87;
     }
-    I->u[1] = (I->u[1] << 1) | tt;
+    I->u[1] <<= 1;
+    I->u[1] |= tt;
 
     xts_uint128_le_to_cpus(I);
 }

either way it generates the exact same asm code

> 
> > +    if (I->u[1] >> 63) {
> > +        I->u[0] ^= 0x87;
> >      }
> > +    I->u[1] = (I->u[1] << 1) | tt;
> > +
> > +    xts_uint128_le_to_cpus(I);
> 
> I think both endianness conversion calls should be flipped. First you
> convert from the buffer byte order (LE) to the CPU byte order so you can
> do the bit shifts, then back to the original byte order (LE).
> 
> Changing this doesn't have any practical effect because both calls
> perform the exact same operation, but it documents better what's going
> on.

Yep, ok

> With this changed,
> 
> Reviewed-by: Alberto Garcia <berto@igalia.com>

Regards,
Daniel
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