> AMD have released an microcode update for affected processors.

I don't think that is correct. AMD has released a microcode update[0] for family 17h models 0x31 and 0xa0, which corresponds to Rome, Castle Peak and Mendocino as per WikiChip [1].

So far, there seems to be no microcode update for Renoir, Grey Hawk, Lucienne, Matisse and Van Gogh. Fortunately, the newly released kernels can and do simply set the chicken bit for those. [2]

[0] https://git.kernel.org/pub/scm/linux/kernel/git/firmware/lin...

[1] https://en.wikichip.org/wiki/amd/cpuid#Family_23_.2817h.29

[2] https://github.com/torvalds/linux/commit/522b1d69219d8f08317...