Comment 3 for bug 1890069

These are old, but might still be true:
https://lists.gnu.org/archive/html/qemu-devel/2013-06/msg04436.html
https://qemu-devel.nongnu.narkive.com/m6BNDsfR/does-qemu-support-mips-smp2-malta-board
https://<email address hidden>/msg10710.html

If that is it then that is a natural limitation.

The only bit I've seen mentioning >2 to work is
https://www.mips.com/blog/how-to-run-smp-linux-in-qemu-on-a-mips64-release-6-cpu/

But the I6400 cpu doesn't exist with newer qemus I7200 does but doesn't work either.
This seems like an upstream feature request to me, but one that was mostly denied 7 years ago (by architecture unable to do so).

Let me know if this is a regression and if I missed something.
Otherwise yeah - you could start a new upstream discussion, but no promises it will end any different than the past ones.