> Date: Fri, 5 Jun 2020 23:27:13 +0200
> From: Christian Weisgerber <na...@mips.inka.de>
> Content-Type: text/plain; charset=us-ascii
> Content-Disposition: inline
>
> Here is a cpu_rnd_messybits() implementation for arm64.
> It reads the virtual counter and xors it with a bit-reversed copy
> of itself.
>
> The virtual counter is used by the only timecounter implementation
> used on arm64, so I assume it is generally available.
>
> It's a 64-bit counter, which we reduce to 32 bits. Since there is
> progressively less entropy in the higher bits of a counter than in
> the lower bits, it intuitively makes sense not just to do hi^lo,
> but to bit-reverse one half in order to extract maximal entropy,
> and on aarch64 bit reversal is a simple instruction.
>
> This patch I have tested!
>
> ok?
>
> Index: arch/arm64/arm64/machdep.c
> ===================================================================
> RCS file: /cvs/src/sys/arch/arm64/arm64/machdep.c,v
> retrieving revision 1.52
> diff -u -p -r1.52 machdep.c
> --- arch/arm64/arm64/machdep.c 31 May 2020 06:23:57 -0000 1.52
> +++ arch/arm64/arm64/machdep.c 5 Jun 2020 20:00:20 -0000
> @@ -1247,12 +1247,3 @@ dumpregs(struct trapframe *frame)
> printf("pc: 0x%016lx\n", frame->tf_elr);
> printf("spsr: 0x%016lx\n", frame->tf_spsr);
> }
> -
> -unsigned int
> -cpu_rnd_messybits(void)
> -{
> - struct timespec ts;
> -
> - nanotime(&ts);
> - return (ts.tv_nsec ^ (ts.tv_sec << 20));
> -}
> Index: arch/arm64/include/cpu.h
> ===================================================================
> RCS file: /cvs/src/sys/arch/arm64/include/cpu.h,v
> retrieving revision 1.17
> diff -u -p -r1.17 cpu.h
> --- arch/arm64/include/cpu.h 31 May 2020 06:23:57 -0000 1.17
> +++ arch/arm64/include/cpu.h 5 Jun 2020 20:32:01 -0000
> @@ -183,7 +183,16 @@ void cpu_boot_secondary_processors(void)
>
> #define curpcb curcpu()->ci_curpcb
>
> -unsigned int cpu_rnd_messybits(void);
> +static inline unsigned int
> +cpu_rnd_messybits(void)
> +{
> + u_int64_t val, rval;
Can you make that uint64_t?
> +
> + __asm volatile("mrs %x0, CNTVCT_EL0; rbit %x1, %x0;"
> + : "=r" (val), "=r" (rval));
Can you use %0 and %1 instead of %x0 and %x1?
> +
> + return (val ^ rval);
> +}
>
> /*
> * Scheduling glue
> --
> Christian "naddy" Weisgerber na...@mips.inka.de
>
>
