[PATCH v2 3/3] RISC-V: Implement archrandom when Zkr is available

[Samuel Ortiz <sameo@rivosinc.com>]

The Zkr extension is ratified and provides 16 bits of entropy seed when
reading the SEED CSR.

We can implement arch_get_random_seed_longs() by doing multiple csrrw to
that CSR and filling an unsigned long with valid entropy bits.

Signed-off-by: Samuel Ortiz <sameo@rivosinc.com>
---
 arch/riscv/include/asm/archrandom.h | 70 +++++++++++++++++++++++++++++
 arch/riscv/include/asm/csr.h        |  9 ++++
 2 files changed, 79 insertions(+)
 create mode 100644 arch/riscv/include/asm/archrandom.h

diff --git a/arch/riscv/include/asm/archrandom.h b/arch/riscv/include/asm/archrandom.h
new file mode 100644
index 000000000000..8987cd0b891d
--- /dev/null
+++ b/arch/riscv/include/asm/archrandom.h
@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Kernel interface for the RISCV arch_random_* functions
+ *
+ * Copyright (c) 2023 by Rivos Inc.
+ *
+ */
+
+#ifndef ASM_RISCV_ARCHRANDOM_H
+#define ASM_RISCV_ARCHRANDOM_H
+
+#include <asm/csr.h>
+
+#define SEED_RETRY_LOOPS 10
+
+static inline bool __must_check csr_seed_long(unsigned long *v)
+{
+	unsigned int retry = SEED_RETRY_LOOPS, valid_seeds = 0;
+	const int needed_seeds = sizeof(long) / sizeof(u16);
+	u16 *entropy = (u16 *)v;
+
+	do {
+		/*
+		 * The SEED CSR (0x015) must be accessed with a read-write
+		 * instruction.
+		 */
+		unsigned long csr_seed = csr_swap(CSR_SEED, 0);
+
+		switch (csr_seed & SEED_OPST_MASK) {
+		case SEED_OPST_ES16:
+			entropy[valid_seeds++] = csr_seed & SEED_ENTROPY_MASK;
+			if (valid_seeds == needed_seeds)
+				return true;
+			break;
+
+		case SEED_OPST_DEAD:
+			pr_err_once("archrandom: Unrecoverable error\n");
+			return false;
+
+		case SEED_OPST_BIST:
+		case SEED_OPST_WAIT:
+		default:
+			continue;
+		}
+	} while (--retry);
+
+	return false;
+}
+
+static inline size_t __must_check arch_get_random_longs(unsigned long *v, size_t max_longs)
+{
+	return 0;
+}
+
+static inline size_t __must_check arch_get_random_seed_longs(unsigned long *v, size_t max_longs)
+{
+	if (!max_longs)
+		return 0;
+
+	/*
+	 * If Zkr is supported and csr_seed_long succeeds, we return one long
+	 * worth of entropy.
+	 */
+	if (riscv_has_extension_likely(RISCV_ISA_EXT_ZKR) && csr_seed_long(v))
+		return 1;
+
+	return 0;
+}
+
+#endif /* ASM_RISCV_ARCHRANDOM_H */
diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
index b98b3b6c9da2..7d0ca9082c66 100644
--- a/arch/riscv/include/asm/csr.h
+++ b/arch/riscv/include/asm/csr.h
@@ -389,6 +389,15 @@
 #define CSR_VTYPE		0xc21
 #define CSR_VLENB		0xc22
 
+/* Scalar Crypto Extension - Entropy */
+#define CSR_SEED		0x015
+#define SEED_OPST_MASK		_AC(0xC0000000, UL)
+#define SEED_OPST_BIST		_AC(0x00000000, UL)
+#define SEED_OPST_WAIT		_AC(0x40000000, UL)
+#define SEED_OPST_ES16		_AC(0x80000000, UL)
+#define SEED_OPST_DEAD		_AC(0xC0000000, UL)
+#define SEED_ENTROPY_MASK	_AC(0xFFFF, UL)
+
 #ifdef CONFIG_RISCV_M_MODE
 # define CSR_STATUS	CSR_MSTATUS
 # define CSR_IE		CSR_MIE
-- 
2.41.0