Re: [PATCH 2/4] crypto: powerpc - Re-enable non-REFLECTed CRCs

[Michael Ellerman]

Daniel Axtens  writes:

> When CRC32c was included in the kernel, Anton ripped out
> the #ifdefs around reflected polynomials, because CRC32c
> is always reflected. However, not all CRCs use reflection
> so we'd like to make it optional.
>
> Restore the REFLECT parts from Anton's original CRC32
> implementation (https://github.com/antonblanchard/crc32-vpmsum)
>
> That implementation is available under GPLv2+, so we're OK
> from a licensing point of view:
> https://github.com/antonblanchard/crc32-vpmsum/blob/master/LICENSE.TXT

It's also written by Anton and copyright IBM, so you (we (IBM)) could
always just relicense it anyway.

So doubly OK IMO.

cheers

[PATCH 2/4] crypto: powerpc - Re-enable non-REFLECTed CRCs

[Daniel Axtens]

When CRC32c was included in the kernel, Anton ripped out
the #ifdefs around reflected polynomials, because CRC32c
is always reflected. However, not all CRCs use reflection
so we'd like to make it optional.

Restore the REFLECT parts from Anton's original CRC32
implementation (https://github.com/antonblanchard/crc32-vpmsum)

That implementation is available under GPLv2+, so we're OK
from a licensing point of view:
https://github.com/antonblanchard/crc32-vpmsum/blob/master/LICENSE.TXT

As CRC32c requires REFLECT, add that #define.

Cc: Anton Blanchard 
Signed-off-by: Daniel Axtens 

---

I compared the disassembly of the CRC32c module on LE before and
after the change, and verified that they were the same.

I verified that the crypto self-tests still pass on LE and BE, and
my tests in patch 4 still pass as well.
---
 arch/powerpc/crypto/crc32-vpmsum_core.S | 31 ++-
 arch/powerpc/crypto/crc32c-vpmsum_asm.S |  1 +
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/crypto/crc32-vpmsum_core.S 
b/arch/powerpc/crypto/crc32-vpmsum_core.S
index 629244ef170e..87fabf4d391a 100644
--- a/arch/powerpc/crypto/crc32-vpmsum_core.S
+++ b/arch/powerpc/crypto/crc32-vpmsum_core.S
@@ -35,7 +35,9 @@
 
.text
 
-#if defined(__BIG_ENDIAN__)
+#if defined(__BIG_ENDIAN__) && defined(REFLECT)
+#define BYTESWAP_DATA
+#elif defined(__LITTLE_ENDIAN__) && !defined(REFLECT)
 #define BYTESWAP_DATA
 #else
 #undef BYTESWAP_DATA
@@ -108,7 +110,11 @@ FUNC_START(CRC_FUNCTION_NAME)
/* Get the initial value into v8 */
vxorv8,v8,v8
MTVRD(v8, R3)
+#ifdef REFLECT
vsldoi  v8,zeroes,v8,8  /* shift into bottom 32 bits */
+#else
+   vsldoi  v8,v8,zeroes,4  /* shift into top 32 bits */
+#endif
 
 #ifdef BYTESWAP_DATA
addis   r3,r2,.byteswap_constant@toc@ha
@@ -354,6 +360,7 @@ FUNC_START(CRC_FUNCTION_NAME)
vxorv6,v6,v14
vxorv7,v7,v15
 
+#ifdef REFLECT
/*
 * vpmsumd produces a 96 bit result in the least significant bits
 * of the register. Since we are bit reflected we have to shift it
@@ -368,6 +375,7 @@ FUNC_START(CRC_FUNCTION_NAME)
vsldoi  v5,v5,zeroes,4
vsldoi  v6,v6,zeroes,4
vsldoi  v7,v7,zeroes,4
+#endif
 
/* xor with last 1024 bits */
lvx v8,0,r4
@@ -511,13 +519,33 @@ FUNC_START(CRC_FUNCTION_NAME)
vsldoi  v1,v0,v0,8
vxorv0,v0,v1/* xor two 64 bit results together */
 
+#ifdef REFLECT
/* shift left one bit */
vspltisb v1,1
vsl v0,v0,v1
+#endif
 
vandv0,v0,mask_64bit
+#ifndef REFLECT
+   /*
+* Now for the Barrett reduction algorithm. The idea is to calculate q,
+* the multiple of our polynomial that we need to subtract. By
+* doing the computation 2x bits higher (ie 64 bits) and shifting the
+* result back down 2x bits, we round down to the nearest multiple.
+*/
+   VPMSUMD(v1,v0,const1)   /* ma */
+   vsldoi  v1,zeroes,v1,8  /* q = floor(ma/(2^64)) */
+   VPMSUMD(v1,v1,const2)   /* qn */
+   vxorv0,v0,v1/* a - qn, subtraction is xor in GF(2) */
 
/*
+* Get the result into r3. We need to shift it left 8 bytes:
+* V0 [ 0 1 2 X ]
+* V0 [ 0 X 2 3 ]
+*/
+   vsldoi  v0,v0,zeroes,8  /* shift result into top 64 bits */
+#else
+   /*
 * The reflected version of Barrett reduction. Instead of bit
 * reflecting our data (which is expensive to do), we bit reflect our
 * constants and our algorithm, which means the intermediate data in
@@ -537,6 +565,7 @@ FUNC_START(CRC_FUNCTION_NAME)
 * V0 [ 0 X 2 3 ]
 */
vsldoi  v0,v0,zeroes,4  /* shift result into top 64 bits of */
+#endif
 
/* Get it into r3 */
MFVRD(R3, v0)
diff --git a/arch/powerpc/crypto/crc32c-vpmsum_asm.S 
b/arch/powerpc/crypto/crc32c-vpmsum_asm.S
index c0d080caefc1..d2bea48051a0 100644
--- a/arch/powerpc/crypto/crc32c-vpmsum_asm.S
+++ b/arch/powerpc/crypto/crc32c-vpmsum_asm.S
@@ -842,4 +842,5 @@
.octa 0x000105ec76f1
 
 #define CRC_FUNCTION_NAME __crc32c_vpmsum
+#define REFLECT
 #include "crc32-vpmsum_core.S"
-- 
2.9.3