Except isn't X86_64 ISA technically property of AMD, not Intel?

I know some of the later stuff (SIMD / AVX / SSE / etc) is Intel, but technically I think they could leave that off and still be successful assuming that would be all that's needed to not be infringing (which I don't know if that's the case or not).

It's x86_64 is so obviously inspired by 32/16-bit x86 that practically speaking, it's patented heavily by both.

Software emulation is fine (otherwise QEMU would be pretty dead), it's just hardware accelerated emulation and actual HW implementations of the ISA that Intel does not like.

There's no real case law on that, unfortunately.

And Intel has been making claims that they could start to litigate. https://www.theregister.co.uk/2017/06/09/intel_sends_arm_a_s...

I know the x86 instruction set up to at least the 486 has had its patents expire. Intel wouldn't win on every claim.