Oof, I butchered his last name, but the quote I found was this:
"Details: Tom Lendacky from AMD writes[4] -- "The idea behind
'virt-ssbd' was to provide an architectural method for a guest to do
SSBD when 'amd-ssbd' isn't present. The 'amd-ssbd' feature will
use
SPEC_CTRL which is intended to not be intercepted and will be fast.
The use of 'virt-ssbd' will always be intercepted and therefore will
not be as fast. So a guest should be presented with 'amd-ssbd', if
available, in preference to 'virt-ssbd'.
It seems for these EPYC CPUs with qemu on 8.1, virt-ssbd isn't an option anymore and
it has to be amd-ssbd for the VM to work?