On 28/04/2025 10:38 pm, H. Peter Anvin wrote: > On April 28, 2025 9:14:45 AM PDT, Linus Torvalds > <torva...@linux-foundation.org> wrote: >> On Mon, 28 Apr 2025 at 00:05, Ingo Molnar <mi...@kernel.org> wrote: >>> And once we remove 486, I think we can do the optimization below to >>> just assume the output doesn't get clobbered by BS*L in the zero-case, >>> right? >> We probably can't, because who knows what "Pentium" CPU's are out there. >> >> Or even if Pentium really does get it right. I doubt we have any >> developers with an original Pentium around. >> >> So just leave the "we don't know what the CPU result is for zero" >> unless we get some kind of official confirmation. >> >> Linus > If anyone knows for sure, it is probably Christian Ludloff. However, there > was a *huge* tightening of the formal ISA when the i686 was introduced > (family=6) and I really believe this was part of it. > > I also really don't trust that family=5 really means conforms to undocumented > P5 behavior, e.g. for Quark.
https://www.sandpile.org/x86/flags.htm That's a lot of "can't even characterise the result" in the P5. Looking at P4 column, that is clearly what the latest SDM has retroactively declared to be architectural. ~Andrew
