Comment by ComputerGuru
5 days ago
Funny. Look up errata AMD-SB-7055: RDSEED Failure on AMD “Zen 5” Processors.
Zen 5 rdrand16/32 return zero with CF=1 on entropy exhaustion and their recommended approach directly leads to the issue you observed: treat all-zero result of rdseed as if cf=0 (failure) and re-roll the dice, effectively recreating the zen 1/zen 2 issue all over again!
They say this might be addressed by a future microcode update… meaning there’s a chance they’ll just patch it to do just that in software. Maybe that’s how they got into this mess in the first place?
Also, am I a complete idiot or is asserting the relative distribution of a mere 64k possible results a rather easy black box validation test that I would’ve assumed they’d be doing? When I used to write cycle-accurate emulators in the past, that would have been an obvious test to include. This isn’t some arcane instruction no one uses or a really complicated case with deep dependency and/or timing issues; it’s like getting rdtsc wrong.
I was going to suggest exactly that, if you're got an RNG, or pretty much anything else for that matter, you need the ability to return some sort of things-went-wrong-somewhere indicator value, and presumably AMD is using 0 to do this. Yes, there's also the CF, but the caller may not be checking that, particularly if it's being done from a HLL.
Has anyone checked whether it can return ~0, (signed) -1, the traditional error-return value?
> Yes, there's also the CF, but the caller may not be checking that, particularly if it's being done from a HLL.
You can’t call a CPU instruction from a high-level language. You would either use inline assembly or call a library function.
Either way, not handling CF=0 would be a bug (in your code or in the library function)