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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