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

[Guo Ren]

On Wed, Jun 28, 2023 at 9:15 AM Samuel Ortiz <sameo at rivosinc.com> wrote:
>
> 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 at 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)
max_longs = 1? needed_seeds only could be 2/4.

> +                               return true;
> +                       break;
> +
> +               case SEED_OPST_DEAD:
> +                       pr_err_once("archrandom: Unrecoverable error\n");
Do we need this pr_err? Could we treat it as a return false? Yes, it's
a hardware problem, but not serious.

> +                       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 )
if (max_longs == 1) ?
> +               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
>

Reviewed-by: Guo Ren <guoren at kernel.org>

-- 
Best Regards
 Guo Ren