Re: [PATCH] RISC-V: Implement archrandom when Zkr is available

[Ben Dooks <ben.dooks@codethink.co.uk>]

On 29/11/2023 16:03, Clément Léger wrote:
> From: Samuel Ortiz <sameo@rivosinc.com>
> 
> From: 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.
> 
> Acked-by: Conor Dooley <conor.dooley@microchip.com>
> Signed-off-by: Samuel Ortiz <sameo@rivosinc.com>
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> 
> ---
> 
> This series depends on "riscv: report more ISA extensions through
> hwprobe" series [1].
> 
> Link: https://lore.kernel.org/lkml/20231114141256.126749-1-cleger@rivosinc.com/ [1]
> ---
>   arch/riscv/include/asm/archrandom.h | 69 +++++++++++++++++++++++++++++
>   arch/riscv/include/asm/csr.h        |  9 ++++
>   2 files changed, 78 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..795837ccb583
> --- /dev/null
> +++ b/arch/riscv/include/asm/archrandom.h
> @@ -0,0 +1,69 @@
> +/* 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 100
> +
> +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 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;

is it worth adding a cpu_relax() here?

> +		}
> +	} 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;

I'm assuming the code will retry if max_longs > 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 306a19a5509c..510014051f5d 100644
> --- a/arch/riscv/include/asm/csr.h
> +++ b/arch/riscv/include/asm/csr.h
> @@ -411,6 +411,15 @@
>   #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

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

https://www.codethink.co.uk/privacy.html