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

[Conor Dooley]

Hey Samuel,

On Tue, Jun 27, 2023 at 04:37:44PM +0200, Samuel Ortiz 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@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/archrandom.h | 66 +++++++++++++++++++++++++++++
>  arch/riscv/include/asm/csr.h        |  9 ++++
>  2 files changed, 75 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..3d01aab2800a
> --- /dev/null
> +++ b/arch/riscv/include/asm/archrandom.h
> @@ -0,0 +1,66 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Kernel interface for the RISCV arch_random_* functions
> + *
> + * Copyright (c) 2022 by Rivos Inc.
> + *
> + */
> +
> +#ifndef ASM_RISCV_ARCHRANDOM_H
> +#define ASM_RISCV_ARCHRANDOM_H
> +
> +#include <asm/csr.h>
> +
> +#define PR_PREFIX "Zkr Extension: "

Does pr_fmt() not work for you?
Also, "Zkr Extension" doesn't really seem super helpful to a punter if
they saw it in a log. Why not s/Zkr Extension/archrandom/, or similar?

> +#define SEED_RETRY_LOOPS 10
> +
> +static inline bool __must_check csr_seed_long(unsigned long *v)
> +{
> +	unsigned int retry = SEED_RETRY_LOOPS;
> +	unsigned int needed_seeds = sizeof(unsigned long) / 2, valid_seeds = 0;
> +	u16 *entropy = (u16 *)v;
> +
> +	do {
> +		/*
> +		 * The SEED CSR (0x015) must be accessed with a read-write
> +		 * instruction. Moreover, implementations must ignore the write
> +		 * value, its purpose is to signal polling for new seed.
> +		 */

What relevance does the second half of this comment have to the kernel?

> +		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(PR_PREFIX "Unrecoverable error\n");
> +			return false;
> +
> +		case SEED_OPST_BIST:
> +			pr_info(PR_PREFIX "On going Built-in Self Test\n");

tiny nit, "On-going"? My OCD is bother by the capitalisation otherwise.

> +			fallthrough;
> +
> +		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)
> +{
> +	return max_longs && riscv_isa_extension_available(NULL, ZKR) && csr_seed_long(v) ? 1 : 0;

Could you please write this in a more readable way, even if that is
going to be a lot more verbose? I know you copied it from x86, but
that's not really an excuse ;)

Also, is there a reason that you opted not to use the alternative-backed
riscv_has_extension_[un]likely() here? The ones backed by alternatives
are preferred if you are not checking on a per-cpu, or once-off basis.

Cheers,
Conor.

> +}
> +
> +#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
> 
> 
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