[PATCH RFC 10/12] RISC-V: crypto: add Zvkb accelerated GCM GHASH implementation

[Heiko Stuebner <heiko@sntech.de>]

From: Heiko Stuebner <heiko.stuebner@vrull.eu>

Add a gcm hash implementation using the Zvkb crypto extension.
It gets possibly registered alongside the Zbc-based variant, with a higher
priority so that the crypto subsystem will be able to select the most
performant variant, but the algorithm itself will still be part of the
crypto selftests that run during registration.

Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
---
 arch/riscv/crypto/Kconfig               |   3 +-
 arch/riscv/crypto/Makefile              |   8 +-
 arch/riscv/crypto/ghash-riscv64-glue.c  | 146 ++++++++++
 arch/riscv/crypto/ghash-riscv64-zvkb.pl | 346 ++++++++++++++++++++++++
 4 files changed, 501 insertions(+), 2 deletions(-)
 create mode 100644 arch/riscv/crypto/ghash-riscv64-zvkb.pl

diff --git a/arch/riscv/crypto/Kconfig b/arch/riscv/crypto/Kconfig
index 010adbbb058a..404fd9b3cb7c 100644
--- a/arch/riscv/crypto/Kconfig
+++ b/arch/riscv/crypto/Kconfig
@@ -4,7 +4,7 @@ menu "Accelerated Cryptographic Algorithms for CPU (riscv)"
 
 config CRYPTO_GHASH_RISCV64
 	tristate "Hash functions: GHASH"
-	depends on 64BIT && RISCV_ISA_ZBC
+	depends on 64BIT && (RISCV_ISA_ZBC || RISCV_ISA_V)
 	select CRYPTO_HASH
 	select CRYPTO_LIB_GF128MUL
 	help
@@ -12,5 +12,6 @@ config CRYPTO_GHASH_RISCV64
 
 	  Architecture: riscv64 using one of:
 	  - ZBC extension
+	  - ZVKB vector crypto extension
 
 endmenu
diff --git a/arch/riscv/crypto/Makefile b/arch/riscv/crypto/Makefile
index 0a158919e9da..8ab9a0ae8f2d 100644
--- a/arch/riscv/crypto/Makefile
+++ b/arch/riscv/crypto/Makefile
@@ -8,6 +8,9 @@ ghash-riscv64-y := ghash-riscv64-glue.o
 ifdef CONFIG_RISCV_ISA_ZBC
 ghash-riscv64-y += ghash-riscv64-zbc.o
 endif
+ifdef CONFIG_RISCV_ISA_V
+ghash-riscv64-y += ghash-riscv64-zvkb.o
+endif
 
 quiet_cmd_perlasm = PERLASM $@
       cmd_perlasm = $(PERL) $(<) void $(@)
@@ -15,4 +18,7 @@ quiet_cmd_perlasm = PERLASM $@
 $(obj)/ghash-riscv64-zbc.S: $(src)/ghash-riscv64-zbc.pl
 	$(call cmd,perlasm)
 
-clean-files += ghash-riscv64-zbc.S
+$(obj)/ghash-riscv64-zvkb.S: $(src)/ghash-riscv64-zvkb.pl
+	$(call cmd,perlasm)
+
+clean-files += ghash-riscv64-zbc.S ghash-riscv64-zvkb.S
diff --git a/arch/riscv/crypto/ghash-riscv64-glue.c b/arch/riscv/crypto/ghash-riscv64-glue.c
index 9802b6718c3c..7376a8a793aa 100644
--- a/arch/riscv/crypto/ghash-riscv64-glue.c
+++ b/arch/riscv/crypto/ghash-riscv64-glue.c
@@ -16,6 +16,7 @@
 #include <linux/crypto.h>
 #include <linux/module.h>
 #include <asm/simd.h>
+#include <asm/vector.h>
 #include <crypto/ghash.h>
 #include <crypto/internal/hash.h>
 #include <crypto/internal/simd.h>
@@ -26,6 +27,10 @@ void gcm_ghash_rv64i_zbc(u64 Xi[2], const u128 Htable[16],
 void gcm_ghash_rv64i_zbc__zbkb(u64 Xi[2], const u128 Htable[16],
 			       const u8 *inp, size_t len);
 
+/* Zvkb (vector crypto with vclmul) based routines. */
+void gcm_ghash_rv64i_zvkb(u64 Xi[2], const u128 Htable[16],
+			  const u8 *inp, size_t len);
+
 struct riscv64_ghash_ctx {
 	void (*ghash_func)(u64 Xi[2], const u128 Htable[16],
 			   const u8 *inp, size_t len);
@@ -51,6 +56,139 @@ static int riscv64_ghash_init(struct shash_desc *desc)
 	return 0;
 }
 
+#ifdef CONFIG_RISCV_ISA_V
+
+#define RISCV64_ZVK_SETKEY(VARIANT, GHASH)				\
+void gcm_init_rv64i_ ## VARIANT(u128 Htable[16], const u64 Xi[2]);	\
+static int riscv64_zvk_ghash_setkey_ ## VARIANT(struct crypto_shash *tfm,	\
+					   const u8 *key,		\
+					   unsigned int keylen)		\
+{									\
+	struct riscv64_ghash_ctx *ctx = crypto_tfm_ctx(crypto_shash_tfm(tfm)); \
+	const u64 k[2] = { cpu_to_be64(((const u64 *)key)[0]),		\
+			   cpu_to_be64(((const u64 *)key)[1]) };	\
+									\
+	if (keylen != GHASH_BLOCK_SIZE)					\
+		return -EINVAL;						\
+									\
+	memcpy(&ctx->key, key, GHASH_BLOCK_SIZE);			\
+	kernel_rvv_begin();						\
+	gcm_init_rv64i_ ## VARIANT(ctx->htable, k);			\
+	kernel_rvv_end();						\
+									\
+	ctx->ghash_func = gcm_ghash_rv64i_ ## GHASH;			\
+									\
+	return 0;							\
+}
+
+static inline void __ghash_block(struct riscv64_ghash_ctx *ctx,
+				 struct riscv64_ghash_desc_ctx *dctx)
+{
+	if (crypto_simd_usable()) {
+		kernel_rvv_begin();
+		ctx->ghash_func(dctx->shash, ctx->htable,
+				dctx->buffer, GHASH_DIGEST_SIZE);
+		kernel_rvv_end();
+	} else {
+		crypto_xor((u8 *)dctx->shash, dctx->buffer, GHASH_BLOCK_SIZE);
+		gf128mul_lle((be128 *)dctx->shash, &ctx->key);
+	}
+}
+
+static inline void __ghash_blocks(struct riscv64_ghash_ctx *ctx,
+				  struct riscv64_ghash_desc_ctx *dctx,
+				  const u8 *src, unsigned int srclen)
+{
+	if (crypto_simd_usable()) {
+		kernel_rvv_begin();
+		ctx->ghash_func(dctx->shash, ctx->htable,
+				src, srclen);
+		kernel_rvv_end();
+	} else {
+		while (srclen >= GHASH_BLOCK_SIZE) {
+			crypto_xor((u8 *)dctx->shash, src, GHASH_BLOCK_SIZE);
+			gf128mul_lle((be128 *)dctx->shash, &ctx->key);
+			srclen -= GHASH_BLOCK_SIZE;
+			src += GHASH_BLOCK_SIZE;
+		}
+	}
+}
+
+static int riscv64_zvk_ghash_update(struct shash_desc *desc,
+			   const u8 *src, unsigned int srclen)
+{
+	unsigned int len;
+	struct riscv64_ghash_ctx *ctx = crypto_tfm_ctx(crypto_shash_tfm(desc->tfm));
+	struct riscv64_ghash_desc_ctx *dctx = shash_desc_ctx(desc);
+
+	if (dctx->bytes) {
+		if (dctx->bytes + srclen < GHASH_DIGEST_SIZE) {
+			memcpy(dctx->buffer + dctx->bytes, src,
+				srclen);
+			dctx->bytes += srclen;
+			return 0;
+		}
+		memcpy(dctx->buffer + dctx->bytes, src,
+			GHASH_DIGEST_SIZE - dctx->bytes);
+
+		__ghash_block(ctx, dctx);
+
+		src += GHASH_DIGEST_SIZE - dctx->bytes;
+		srclen -= GHASH_DIGEST_SIZE - dctx->bytes;
+		dctx->bytes = 0;
+	}
+	len = srclen & ~(GHASH_DIGEST_SIZE - 1);
+
+	if (len) {
+		__ghash_blocks(ctx, dctx, src, len);
+		src += len;
+		srclen -= len;
+	}
+
+	if (srclen) {
+		memcpy(dctx->buffer, src, srclen);
+		dctx->bytes = srclen;
+	}
+	return 0;
+}
+
+static int riscv64_zvk_ghash_final(struct shash_desc *desc, u8 *out)
+{
+	int i;
+	struct riscv64_ghash_ctx *ctx = crypto_tfm_ctx(crypto_shash_tfm(desc->tfm));
+	struct riscv64_ghash_desc_ctx *dctx = shash_desc_ctx(desc);
+
+	if (dctx->bytes) {
+		for (i = dctx->bytes; i < GHASH_DIGEST_SIZE; i++)
+			dctx->buffer[i] = 0;
+		__ghash_block(ctx, dctx);
+		dctx->bytes = 0;
+	}
+	memcpy(out, dctx->shash, GHASH_DIGEST_SIZE);
+	return 0;
+}
+
+RISCV64_ZVK_SETKEY(zvkb, zvkb);
+struct shash_alg riscv64_zvkb_ghash_alg = {
+	.digestsize = GHASH_DIGEST_SIZE,
+	.init = riscv64_ghash_init,
+	.update = riscv64_zvk_ghash_update,
+	.final = riscv64_zvk_ghash_final,
+	.setkey = riscv64_zvk_ghash_setkey_zvkb,
+	.descsize = sizeof(struct riscv64_ghash_desc_ctx)
+		    + sizeof(struct ghash_desc_ctx),
+	.base = {
+		 .cra_name = "ghash",
+		 .cra_driver_name = "riscv64_zvkb_ghash",
+		 .cra_priority = 300,
+		 .cra_blocksize = GHASH_BLOCK_SIZE,
+		 .cra_ctxsize = sizeof(struct riscv64_ghash_ctx),
+		 .cra_module = THIS_MODULE,
+	},
+};
+
+#endif /* CONFIG_RISCV_ISA_V */
+
 #ifdef CONFIG_RISCV_ISA_ZBC
 
 #define RISCV64_ZBC_SETKEY(VARIANT, GHASH)				\
@@ -241,6 +379,14 @@ static int __init riscv64_ghash_mod_init(void)
 	}
 #endif
 
+#ifdef CONFIG_RISCV_ISA_V
+	if (riscv_isa_extension_available(NULL, ZVKB)) {
+		ret = riscv64_ghash_register(&riscv64_zvkb_ghash_alg);
+		if (ret < 0)
+			return ret;
+	}
+#endif
+
 	return 0;
 }
 
diff --git a/arch/riscv/crypto/ghash-riscv64-zvkb.pl b/arch/riscv/crypto/ghash-riscv64-zvkb.pl
new file mode 100644
index 000000000000..8450e850108b
--- /dev/null
+++ b/arch/riscv/crypto/ghash-riscv64-zvkb.pl
@@ -0,0 +1,346 @@
+#! /usr/bin/env perl
+# Copyright 2023 The OpenSSL Project Authors. All Rights Reserved.
+#
+# Licensed under the Apache License 2.0 (the "License").  You may not use
+# this file except in compliance with the License.  You can obtain a copy
+# in the file LICENSE in the source distribution or at
+# https://www.openssl.org/source/license.html
+
+use strict;
+use warnings;
+
+use FindBin qw($Bin);
+use lib "$Bin";
+use lib "$Bin/../../perlasm";
+use riscv;
+
+# $output is the last argument if it looks like a file (it has an extension)
+# $flavour is the first argument if it doesn't look like a file
+my $output = $#ARGV >= 0 && $ARGV[$#ARGV] =~ m|\.\w+$| ? pop : undef;
+my $flavour = $#ARGV >= 0 && $ARGV[0] !~ m|\.| ? shift : undef;
+
+$output and open STDOUT,">$output";
+
+my $code=<<___;
+.text
+___
+
+################################################################################
+# void gcm_init_rv64i_zvkg(u128 Htable[16], const u64 H[2]);
+#
+# input:	H: 128-bit H - secret parameter E(K, 0^128)
+# output:	Htable: Preprocessed key data for gcm_gmult_rv64i_zvkb and
+#                       gcm_ghash_rv64i_zvkb
+{
+my ($Htable,$H,$TMP0,$TMP1,$TMP2) = ("a0","a1","t0","t1","t2");
+my ($V0,$V1,$V2,$V3,$V4,$V5,$V6) = ("v0","v1","v2","v3","v4","v5","v6");
+
+$code .= <<___;
+.p2align 3
+.globl gcm_init_rv64i_zvkb
+.type gcm_init_rv64i_zvkb,\@function
+gcm_init_rv64i_zvkb:
+    # Load/store data in reverse order.
+    # This is needed as a part of endianness swap.
+    add $H, $H, 8
+    li $TMP0, -8
+    li $TMP1, 63
+    la $TMP2, Lpolymod
+
+    @{[vsetivli__x0_2_e64_m1_ta_ma]} # vsetivli x0, 2, e64, m1, ta, ma
+
+    @{[vlse64_v  $V1, $H, $TMP0]}    # vlse64.v v1, (a1), t0
+    @{[vle64_v $V2, $TMP2]}          # vle64.v v2, (t2)
+
+    # Shift one left and get the carry bits.
+    @{[vsrl_vx $V3, $V1, $TMP1]}     # vsrl.vx v3, v1, t1
+    @{[vsll_vi $V1, $V1, 1]}         # vsll.vi v1, v1, 1
+
+    # Use the fact that the polynomial degree is no more than 128,
+    # i.e. only the LSB of the upper half could be set.
+    # Thanks to we don't need to do the full reduction here.
+    # Instead simply subtract the reduction polynomial.
+    # This idea was taken from x86 ghash implementation in OpenSSL.
+    @{[vslideup_vi $V4, $V3, 1]}     # vslideup.vi v4, v3, 1
+    @{[vslidedown_vi $V3, $V3, 1]}   # vslidedown.vi v3, v3, 1
+
+    @{[vmv_v_i $V0, 2]}              # vmv.v.i v0, 2
+    @{[vor_vv_v0t $V1, $V1, $V4]}    # vor.vv v1, v1, v4, v0.t
+
+    # Need to set the mask to 3, if the carry bit is set.
+    # Not sure if there is a better way of doing this.
+    @{[vmv_v_v $V0, $V3]}            # vmv.v.v v0, v3
+    @{[vmv_v_i $V3, 0]}              # vmv.v.i v3, 0
+    @{[vmerge_vim $V3, $V3, 3]}      # vmerge.vim v3, v3, 3, v0
+    @{[vmv_v_v $V0, $V3]}            # vmv.v.v v0, v3
+
+    @{[vxor_vv_v0t $V1, $V1, $V2]}   # vxor.vv v1, v1, v2, v0.t
+
+    @{[vse64_v $V1, $Htable]}        # vse64.v v1, (a0)
+    ret
+.size gcm_init_rv64i_zvkb,.-gcm_init_rv64i_zvkb
+___
+}
+
+################################################################################
+# void gcm_gmult_rv64i_zvkb(u64 Xi[2], const u128 Htable[16]);
+#
+# input:	Xi: current hash value
+#		Htable: preprocessed H
+# output:	Xi: next hash value Xi = (Xi * H mod f)
+{
+my ($Xi,$Htable,$TMP0,$TMP1,$TMP2,$TMP3,$TMP4) = ("a0","a1","t0","t1","t2","t3","t4");
+my ($V0,$V1,$V2,$V3,$V4,$V5,$V6) = ("v0","v1","v2","v3","v4","v5","v6");
+
+$code .= <<___;
+.text
+.p2align 3
+.globl gcm_gmult_rv64i_zvkb
+.type gcm_gmult_rv64i_zvkb,\@function
+gcm_gmult_rv64i_zvkb:
+    ld $TMP0, ($Htable)
+    ld $TMP1, 8($Htable)
+    li $TMP2, 63
+    la $TMP3, Lpolymod
+    ld $TMP3, 8($TMP3)
+
+    # Load/store data in reverse order.
+    # This is needed as a part of endianness swap.
+    add $Xi, $Xi, 8
+    li $TMP4, -8
+
+    @{[vsetivli__x0_2_e64_m1_ta_ma]} # vsetivli x0, 2, e64, m1, ta, ma
+
+    @{[vlse64_v $V5, $Xi, $TMP4]}    # vlse64.v v5, (a0), t4
+    @{[vrev8_v $V5, $V5]}            # vrev8.v v5, v5
+
+    # Multiplication
+
+    # Do two 64x64 multiplications in one go to save some time
+    # and simplify things.
+
+    # A = a1a0 (t1, t0)
+    # B = b1b0 (v5)
+    # C = c1c0 (256 bit)
+    # c1 = a1b1 + (a0b1)h + (a1b0)h
+    # c0 = a0b0 + (a0b1)l + (a1b0)h
+
+    # v1 = (a0b1)l,(a0b0)l
+    @{[vclmul_vx $V1, $V5, $TMP0]}   # vclmul.vx v1, v5, t0
+    # v3 = (a0b1)h,(a0b0)h
+    @{[vclmulh_vx $V3, $V5, $TMP0]}  # vclmulh.vx v3, v5, t0
+
+    # v4 = (a1b1)l,(a1b0)l
+    @{[vclmul_vx $V4, $V5, $TMP1]}   # vclmul.vx v4, v5, t1
+    # v2 = (a1b1)h,(a1b0)h
+    @{[vclmulh_vx $V2, $V5, $TMP1]}   # vclmulh.vx v2, v5, t1
+
+    # Is there a better way to do this?
+    # Would need to swap the order of elements within a vector register.
+    @{[vslideup_vi $V5, $V3, 1]}     # vslideup.vi v5, v3, 1
+    @{[vslideup_vi $V6, $V4, 1]}     # vslideup.vi v6, v4, 1
+    @{[vslidedown_vi $V3, $V3, 1]}   # vslidedown.vi v3, v3, 1
+    @{[vslidedown_vi $V4, $V4, 1]}   # vslidedown.vi v4, v4, 1
+
+    @{[vmv_v_i $V0, 1]}              # vmv.v.i v0, 1
+    # v2 += (a0b1)h
+    @{[vxor_vv_v0t $V2, $V2, $V3]}   # vxor.vv v2, v2, v3, v0.t
+    # v2 += (a1b1)l
+    @{[vxor_vv_v0t $V2, $V2, $V4]}   # vxor.vv v2, v2, v4, v0.t
+
+    @{[vmv_v_i $V0, 2]}              # vmv.v.i v0, 2
+    # v1 += (a0b0)h,0
+    @{[vxor_vv_v0t $V1, $V1, $V5]}   # vxor.vv v1, v1, v5, v0.t
+    # v1 += (a1b0)l,0
+    @{[vxor_vv_v0t $V1, $V1, $V6]}   # vxor.vv v1, v1, v6, v0.t
+
+    # Now the 256bit product should be stored in (v2,v1)
+    # v1 = (a0b1)l + (a0b0)h + (a1b0)l, (a0b0)l
+    # v2 = (a1b1)h, (a1b0)h + (a0b1)h + (a1b1)l
+
+    # Reduction
+    # Let C := A*B = c3,c2,c1,c0 = v2[1],v2[0],v1[1],v1[0]
+    # This is a slight variation of the Gueron's Montgomery reduction.
+    # The difference being the order of some operations has been changed,
+    # to make a better use of vclmul(h) instructions.
+
+    # First step:
+    # c1 += (c0 * P)l
+    # vmv.v.i v0, 2
+    @{[vslideup_vi_v0t $V3, $V1, 1]} # vslideup.vi v3, v1, 1, v0.t
+    @{[vclmul_vx_v0t $V3, $V3, $TMP3]} # vclmul.vx v3, v3, t3, v0.t
+    @{[vxor_vv_v0t $V1, $V1, $V3]}   # vxor.vv v1, v1, v3, v0.t
+
+    # Second step:
+    # D = d1,d0 is final result
+    # We want:
+    # m1 = c1 + (c1 * P)h
+    # m0 = (c1 * P)l + (c0 * P)h + c0
+    # d1 = c3 + m1
+    # d0 = c2 + m0
+
+    #v3 = (c1 * P)l, 0
+    @{[vclmul_vx_v0t $V3, $V1, $TMP3]} # vclmul.vx v3, v1, t3, v0.t
+    #v4 = (c1 * P)h, (c0 * P)h
+    @{[vclmulh_vx $V4, $V1, $TMP3]}   # vclmulh.vx v4, v1, t3
+
+    @{[vmv_v_i $V0, 1]}              # vmv.v.i v0, 1
+    @{[vslidedown_vi $V3, $V3, 1]}   # vslidedown.vi v3, v3, 1
+
+    @{[vxor_vv $V1, $V1, $V4]}       # vxor.vv v1, v1, v4
+    @{[vxor_vv_v0t $V1, $V1, $V3]}   # vxor.vv v1, v1, v3, v0.t
+
+    # XOR in the upper upper part of the product
+    @{[vxor_vv $V2, $V2, $V1]}       # vxor.vv v2, v2, v1
+
+    @{[vrev8_v $V2, $V2]}            # vrev8.v v2, v2
+    @{[vsse64_v $V2, $Xi, $TMP4]}    # vsse64.v v2, (a0), t4
+    ret
+.size gcm_gmult_rv64i_zvkb,.-gcm_gmult_rv64i_zvkb
+___
+}
+
+################################################################################
+# void gcm_ghash_rv64i_zvkb(u64 Xi[2], const u128 Htable[16],
+#                           const u8 *inp, size_t len);
+#
+# input:	Xi: current hash value
+#		Htable: preprocessed H
+#		inp: pointer to input data
+#		len: length of input data in bytes (mutiple of block size)
+# output:	Xi: Xi+1 (next hash value Xi)
+{
+my ($Xi,$Htable,$inp,$len,$TMP0,$TMP1,$TMP2,$TMP3,$M8,$TMP5,$TMP6) = ("a0","a1","a2","a3","t0","t1","t2","t3","t4","t5","t6");
+my ($V0,$V1,$V2,$V3,$V4,$V5,$V6,$Vinp) = ("v0","v1","v2","v3","v4","v5","v6","v7");
+
+$code .= <<___;
+.p2align 3
+.globl gcm_ghash_rv64i_zvkb
+.type gcm_ghash_rv64i_zvkb,\@function
+gcm_ghash_rv64i_zvkb:
+    ld $TMP0, ($Htable)
+    ld $TMP1, 8($Htable)
+    li $TMP2, 63
+    la $TMP3, Lpolymod
+    ld $TMP3, 8($TMP3)
+
+    # Load/store data in reverse order.
+    # This is needed as a part of endianness swap.
+    add $Xi, $Xi, 8
+    add $inp, $inp, 8
+    li $M8, -8
+
+    @{[vsetivli__x0_2_e64_m1_ta_ma]} # vsetivli x0, 2, e64, m1, ta, ma
+
+    @{[vlse64_v $V5, $Xi, $M8]}      # vlse64.v v5, (a0), t4
+
+Lstep:
+    # Read input data
+    @{[vlse64_v $Vinp, $inp, $M8]}   # vle64.v v0, (a2)
+    add $inp, $inp, 16
+    add $len, $len, -16
+    # XOR them into Xi
+    @{[vxor_vv $V5, $V5, $Vinp]}       # vxor.vv v0, v0, v1
+
+    @{[vrev8_v $V5, $V5]}            # vrev8.v v5, v5
+
+    # Multiplication
+
+    # Do two 64x64 multiplications in one go to save some time
+    # and simplify things.
+
+    # A = a1a0 (t1, t0)
+    # B = b1b0 (v5)
+    # C = c1c0 (256 bit)
+    # c1 = a1b1 + (a0b1)h + (a1b0)h
+    # c0 = a0b0 + (a0b1)l + (a1b0)h
+
+    # v1 = (a0b1)l,(a0b0)l
+    @{[vclmul_vx $V1, $V5, $TMP0]}   # vclmul.vx v1, v5, t0
+    # v3 = (a0b1)h,(a0b0)h
+    @{[vclmulh_vx $V3, $V5, $TMP0]}  # vclmulh.vx v3, v5, t0
+
+    # v4 = (a1b1)l,(a1b0)l
+    @{[vclmul_vx $V4, $V5, $TMP1]}   # vclmul.vx v4, v5, t1
+    # v2 = (a1b1)h,(a1b0)h
+    @{[vclmulh_vx $V2, $V5, $TMP1]}   # vclmulh.vx v2, v5, t1
+
+    # Is there a better way to do this?
+    # Would need to swap the order of elements within a vector register.
+    @{[vslideup_vi $V5, $V3, 1]}     # vslideup.vi v5, v3, 1
+    @{[vslideup_vi $V6, $V4, 1]}     # vslideup.vi v6, v4, 1
+    @{[vslidedown_vi $V3, $V3, 1]}   # vslidedown.vi v3, v3, 1
+    @{[vslidedown_vi $V4, $V4, 1]}   # vslidedown.vi v4, v4, 1
+
+    @{[vmv_v_i $V0, 1]}              # vmv.v.i v0, 1
+    # v2 += (a0b1)h
+    @{[vxor_vv_v0t $V2, $V2, $V3]}   # vxor.vv v2, v2, v3, v0.t
+    # v2 += (a1b1)l
+    @{[vxor_vv_v0t $V2, $V2, $V4]}   # vxor.vv v2, v2, v4, v0.t
+
+    @{[vmv_v_i $V0, 2]}              # vmv.v.i v0, 2
+    # v1 += (a0b0)h,0
+    @{[vxor_vv_v0t $V1, $V1, $V5]}   # vxor.vv v1, v1, v5, v0.t
+    # v1 += (a1b0)l,0
+    @{[vxor_vv_v0t $V1, $V1, $V6]}   # vxor.vv v1, v1, v6, v0.t
+
+    # Now the 256bit product should be stored in (v2,v1)
+    # v1 = (a0b1)l + (a0b0)h + (a1b0)l, (a0b0)l
+    # v2 = (a1b1)h, (a1b0)h + (a0b1)h + (a1b1)l
+
+    # Reduction
+    # Let C := A*B = c3,c2,c1,c0 = v2[1],v2[0],v1[1],v1[0]
+    # This is a slight variation of the Gueron's Montgomery reduction.
+    # The difference being the order of some operations has been changed,
+    # to make a better use of vclmul(h) instructions.
+
+    # First step:
+    # c1 += (c0 * P)l
+    # vmv.v.i v0, 2
+    @{[vslideup_vi_v0t $V3, $V1, 1]} # vslideup.vi v3, v1, 1, v0.t
+    @{[vclmul_vx_v0t $V3, $V3, $TMP3]} # vclmul.vx v3, v3, t3, v0.t
+    @{[vxor_vv_v0t $V1, $V1, $V3]}   # vxor.vv v1, v1, v3, v0.t
+
+    # Second step:
+    # D = d1,d0 is final result
+    # We want:
+    # m1 = c1 + (c1 * P)h
+    # m0 = (c1 * P)l + (c0 * P)h + c0
+    # d1 = c3 + m1
+    # d0 = c2 + m0
+
+    #v3 = (c1 * P)l, 0
+    @{[vclmul_vx_v0t $V3, $V1, $TMP3]} # vclmul.vx v3, v1, t3, v0.t
+    #v4 = (c1 * P)h, (c0 * P)h
+    @{[vclmulh_vx $V4, $V1, $TMP3]}   # vclmulh.vx v4, v1, t3
+
+    @{[vmv_v_i $V0, 1]}              # vmv.v.i v0, 1
+    @{[vslidedown_vi $V3, $V3, 1]}   # vslidedown.vi v3, v3, 1
+
+    @{[vxor_vv $V1, $V1, $V4]}       # vxor.vv v1, v1, v4
+    @{[vxor_vv_v0t $V1, $V1, $V3]}   # vxor.vv v1, v1, v3, v0.t
+
+    # XOR in the upper upper part of the product
+    @{[vxor_vv $V2, $V2, $V1]}       # vxor.vv v2, v2, v1
+
+    @{[vrev8_v $V5, $V2]}            # vrev8.v v2, v2
+
+    bnez $len, Lstep
+
+    @{[vsse64_v $V5, $Xi, $M8]}    # vsse64.v v2, (a0), t4
+    ret
+.size gcm_ghash_rv64i_zvkb,.-gcm_ghash_rv64i_zvkb
+___
+}
+
+$code .= <<___;
+.p2align 4
+Lpolymod:
+        .dword 0x0000000000000001
+        .dword 0xc200000000000000
+.size Lpolymod,.-Lpolymod
+___
+
+print $code;
+
+close STDOUT or die "error closing STDOUT: $!";
-- 
2.39.0