linux-kernel - RE: [PATCH RFC v0] random: block in /dev/urandom

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Date:   Mon, 14 Feb 2022 14:57:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Geert Uytterhoeven' <geert@...ux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
CC:     Joshua Kinard <kumba@...too.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Eric Biggers <ebiggers@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Andy Lutomirski" <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Lennart Poettering" <mzxreary@...inter.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Theodore Ts'o <tytso@....edu>
Subject: RE: [PATCH RFC v0] random: block in /dev/urandom

From: Geert Uytterhoeven
> Sent: 14 February 2022 14:26
...
> I'm afraid you missed one important detail.  You wrote:
> 
> > On every platform, random_get_entropy() is connected to get_cycles(),
> > except for three: m68k, MIPS, and RISC-V.
> 
> The default implementation in include/asm-generic/timex.h is:
> 
>     static inline cycles_t get_cycles(void)
>     {
>             return 0;
>     }
> 
> Several architectures do not implement get_cycles(), or implement it
> with a variant that's very similar or identical to the generic version.

Add to the list nios2 and old x86 (I think rdtsc is a pentium instruction)
I can't see it in my 386 book, and i don't think 486 added it.

I'm not sure if/when sparc added one.
I don't remember it being there in the late 1980s.

nios2 (soft cpu on Altera/Intel fpga) is annoying.
There is a 'read control register' instruction and plenty of space ones.
But you can't define your own and one isn't a clock counter.
You can add one as the result of a custom instruction.
(Even the same custom instruction that does byteswap.)

	David

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