crypto: curve25519 - x86_64 library and KPP implementations

This implementation is the fastest available x86_64 implementation, and
unlike Sandy2x, it doesn't requie use of the floating point registers at
all. Instead it makes use of BMI2 and ADX, available on recent
microarchitectures. The implementation was written by Armando
Faz-Hernández with contributions (upstream) from Samuel Neves and me,
in addition to further changes in the kernel implementation from us.
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: NSamuel Neves <sneves@dei.uc.pt>
Co-developed-by: NSamuel Neves <sneves@dei.uc.pt>
[ardb: - move to arch/x86/crypto
       - wire into lib/crypto framework
       - implement crypto API KPP hooks ]
Signed-off-by: NArd Biesheuvel <ardb@kernel.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

arch/x86/crypto/Makefile

@@ -39,6 +39,7 @@ obj-$(CONFIG_CRYPTO_AEGIS128_AESNI_SSE2) += aegis128-aesni.o
 
 obj-$(CONFIG_CRYPTO_NHPOLY1305_SSE2) += nhpoly1305-sse2.o
 obj-$(CONFIG_CRYPTO_NHPOLY1305_AVX2) += nhpoly1305-avx2.o
+obj-$(CONFIG_CRYPTO_CURVE25519_X86) += curve25519-x86_64.o
 
 # These modules require assembler to support AVX.
 ifeq ($(avx_supported),yes)

crypto/Kconfig

@@ -269,6 +269,12 @@ config CRYPTO_CURVE25519
 	select CRYPTO_KPP
 	select CRYPTO_LIB_CURVE25519_GENERIC
 
+config CRYPTO_CURVE25519_X86
+	tristate "x86_64 accelerated Curve25519 scalar multiplication library"
+	depends on X86 && 64BIT
+	select CRYPTO_LIB_CURVE25519_GENERIC
+	select CRYPTO_ARCH_HAVE_LIB_CURVE25519
+
 comment "Authenticated Encryption with Associated Data"
 
 config CRYPTO_CCM