Patch "lib/mpi: Fix umul_ppmm() for MIPS64r6" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

lib/mpi: Fix umul_ppmm() for MIPS64r6

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
lib-mpi-fix-umul_ppmm-for-mips64r6.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: James Hogan <[email protected]>
Date: Tue, 5 Dec 2017 23:31:35 +0000
Subject: lib/mpi: Fix umul_ppmm() for MIPS64r6

From: James Hogan <[email protected]>

[ Upstream commit bbc25bee37d2b32cf3a1fab9195b6da3a185614a ]

Current MIPS64r6 toolchains aren't able to generate efficient DMULU/DMUHU based code for the C implementation of umul_ppmm(), which performs an unsigned 64 x 64 bit multiply and returns the upper and lower 64-bit halves of the 128-bit result. Instead it widens the 64-bit inputs to 128-bits and emits a __multi3 intrinsic call to perform a 128 x 128 multiply. This is both inefficient, and it results in a link error since we don't include __multi3 in MIPS linux.

For example commit 90a53e4432b1 ("cfg80211: implement regdb signature checking") merged in v4.15-rc1 recently broke the 64r6_defconfig and 64r6el_defconfig builds by indirectly selecting MPILIB. The same build errors can be reproduced on older kernels by enabling e.g. CRYPTO_RSA:

lib/mpi/generic_mpih-mul1.o: In function `mpihelp_mul_1':
lib/mpi/generic_mpih-mul1.c:50: undefined reference to __multi3' lib/mpi/generic_mpih-mul2.o: In function mpihelp_addmul_1':
lib/mpi/generic_mpih-mul2.c:49: undefined reference to __multi3' lib/mpi/generic_mpih-mul3.o: In function mpihelp_submul_1':
lib/mpi/generic_mpih-mul3.c:49: undefined reference to __multi3' lib/mpi/mpih-div.o In function mpihelp_divrem':
lib/mpi/mpih-div.c:205: undefined reference to __multi3' lib/mpi/mpih-div.c:142: undefined reference to __multi3'

Therefore add an efficient MIPS64r6 implementation of umul_ppmm() using inline assembly and the DMULU/DMUHU instructions, to prevent __multi3 calls being emitted.

---
 lib/mpi/longlong.h |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

@@ -671,7 +671,23 @@ do {						\
 	**************  MIPS/64  **************
 	***************************************/
 #if (defined(__mips) && __mips >= 3) && W_TYPE_SIZE == 64
-#if (__GNUC__ >= 5) || (__GNUC__ >= 4 && __GNUC_MINOR__ >= 4)
+#if defined(__mips_isa_rev) && __mips_isa_rev >= 6
+/*
+ * GCC ends up emitting a __multi3 intrinsic call for MIPS64r6 with the plain C
+ * code below, so we special case MIPS64r6 until the compiler can do better.
+ */
+#define umul_ppmm(w1, w0, u, v)						\
+do {									\
+	__asm__ ("dmulu %0,%1,%2"					\
+		 : "=d" ((UDItype)(w0))					\
+		 : "d" ((UDItype)(u)),					\
+		   "d" ((UDItype)(v)));					\
+	__asm__ ("dmuhu %0,%1,%2"					\
+		 : "=d" ((UDItype)(w1))					\
+		 : "d" ((UDItype)(u)),					\
+		   "d" ((UDItype)(v)));					\
+} while (0)
+#elif (__GNUC__ >= 5) || (__GNUC__ >= 4 && __GNUC_MINOR__ >= 4)
 #define umul_ppmm(w1, w0, u, v) \
 do {									\
 	typedef unsigned int __ll_UTItype __attribute__((mode(TI)));	\

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/lib-mpi-fix-umul_ppmm-for-mips64r6.patch