I don't think so, no. If it has microcode it's probably burned into sequencer tables, not updatable. I was kind of hoping Apple would have some chicken bit register up their sleeve as a last resource fix (e.g. "trap on instruction encodings matching this mask"), but given that they seem to have no useful mitigation for it, I don't think they do.

Is it possible Apple have the silicon functionality to fix this, but have decided it isn't worth fixing?

After all, process isolation between cooperating processes is nearly impossible to do. If Apple close this loophole, there will be other lower bandwidth side channels like spinning up the fan in Morse code and the other process notices the clock speed scaling up and down...

It doesn't really make sense not to fix it if they can in fact do so easily.

Except software-silicon patches usually have a limited number of filters, patch slots, etc. Might not be worth using one for this.

They're using zero so far [0], and until they need it for something else it wouldn't make sense not to use it for this. The CPU tunables aren't fuses or anything, the OS configures them (m1n1 in our case)

[0] https://github.com/AsahiLinux/m1n1/blob/main/src/chickens.c