[PATCH v5 04/11] mips: use fallback for random_get_entropy() instead of just c0 random

[Thomas Bogendoerfer]

On Tue, Apr 19, 2022 at 01:16:43PM +0200, Jason A. Donenfeld wrote:
> For situations in which we don't have a c0 counter register available,
> we've been falling back to reading the c0 "random" register, which is
> usually bounded by the amount of TLB entries and changes every other
> cycle or so. This means it wraps extremely often. We can do better by
> combining this fast-changing counter with a potentially slower-changing
> counter from random_get_entropy_fallback() in the more significant bits.
> This commit combines the two, taking into account that the changing bits
> are in a different bit position depending on the CPU model. In addition,
> we previously were falling back to 0 for ancient CPUs that Linux does
> not support anyway; remove that dead path entirely.
> 
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Thomas Bogendoerfer <tsbogend at alpha.franken.de>
> Cc: Maciej W. Rozycki <macro at orcam.me.uk>
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
> ---
> ThomasB - I dropped your Ack from v4, because this is pretty different
> from v4 now.
> 
> Maciej - you mentioned you had a test rig. Think you could provide a
> "Tested-by" if this approach works?
> 
>  arch/mips/include/asm/timex.h | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/mips/include/asm/timex.h b/arch/mips/include/asm/timex.h
> index b05bb70a2e46..e3f5460a923b 100644
> --- a/arch/mips/include/asm/timex.h
> +++ b/arch/mips/include/asm/timex.h
> @@ -80,21 +80,19 @@ static inline cycles_t get_cycles(void)
>  /*
>   * Like get_cycles - but where c0_count is not available we desperately
>   * use c0_random in an attempt to get at least a little bit of entropy.
> - *
> - * R6000 and R6000A neither have a count register nor a random register.
> - * That leaves no entropy source in the CPU itself.
>   */
>  static inline unsigned long random_get_entropy(void)
>  {
> -	unsigned int prid = read_c0_prid();
> -	unsigned int imp = prid & PRID_IMP_MASK;
> +	unsigned int c0_random;
>  
> -	if (can_use_mips_counter(prid))
> +	if (can_use_mips_counter(read_c0_prid()))
>  		return read_c0_count();
> -	else if (likely(imp != PRID_IMP_R6000 && imp != PRID_IMP_R6000A))
> -		return read_c0_random();
> +
> +	if (cpu_has_3kex)
> +		c0_random = (read_c0_random() >> 8) & 0x3f;
>  	else
> -		return 0;	/* no usable register */
> +		c0_random = read_c0_random() & 0x3f;
> +	return (random_get_entropy_fallback() << 6) | (0x3f - c0_random);
>  }
>  #define random_get_entropy random_get_entropy
>  
> -- 
> 2.35.1

Acked-by: Thomas Bogendoerfer <tsbogend at alpha.franken.de>

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]