crypto: aesni - fix counter overflow handling in "by8" variant

The "by8" CTR AVX implementation fails to propperly handle counter
overflows. That was the reason it got disabled in commit 7da4b29d496b
("crypto: aesni - disable "by8" AVX CTR optimization").

Fix the overflow handling by incrementing the counter block as a double
quad word, i.e. a 128 bit, and testing for overflows afterwards. We need
to use VPTEST to do so as VPADD* does not set the flags itself and
silently drops the carry bit.

As this change adds branches to the hot path, minor performance
regressions  might be a side effect. But, OTOH, we now have a conforming
implementation -- the preferable goal.

A tcrypt test on a SandyBridge system (i7-2620M) showed almost identical
numbers for the old and this version with differences within the noise
range. A dm-crypt test with the fixed version gave even slightly better
results for this version. So the performance impact might not be as big
as expected.

Tested-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Chandramouli Narayanan <mouli@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/x86/crypto/aes_ctrby8_avx-x86_64.S

@@ -108,6 +108,10 @@
 
 byteswap_const:
 	.octa 0x000102030405060708090A0B0C0D0E0F
+ddq_low_msk:
+	.octa 0x0000000000000000FFFFFFFFFFFFFFFF
+ddq_high_add_1:
+	.octa 0x00000000000000010000000000000000
 ddq_add_1:
 	.octa 0x00000000000000000000000000000001
 ddq_add_2:
@@ -169,7 +173,12 @@ ddq_add_8:
 	.rept (by - 1)
 		club DDQ_DATA, i
 		club XDATA, i
-		vpaddd	var_ddq_add(%rip), xcounter, var_xdata
+		vpaddq	var_ddq_add(%rip), xcounter, var_xdata
+		vptest	ddq_low_msk(%rip), var_xdata
+		jnz 1f
+		vpaddq	ddq_high_add_1(%rip), var_xdata, var_xdata
+		vpaddq	ddq_high_add_1(%rip), xcounter, xcounter
+		1:
 		vpshufb	xbyteswap, var_xdata, var_xdata
 		.set i, (i +1)
 	.endr
@@ -178,7 +187,11 @@ ddq_add_8:
 
 	vpxor	xkey0, xdata0, xdata0
 	club DDQ_DATA, by
-	vpaddd	var_ddq_add(%rip), xcounter, xcounter
+	vpaddq	var_ddq_add(%rip), xcounter, xcounter
+	vptest	ddq_low_msk(%rip), xcounter
+	jnz	1f
+	vpaddq	ddq_high_add_1(%rip), xcounter, xcounter
+	1:
 
 	.set i, 1
 	.rept (by - 1)