AMD guidance: https://developer.amd.com/wp-content/resources/124441_AMD64_... (setting an CPU-specific MSR and it's done for current CPUs, no microcode updates required.) https://www.amd.com/en/corporate/security-updates has : "We have not identified any AMD x86 products susceptible to the Variant 3a vulnerability in our analysis to-date."
Some AMD processors only support a non-architectural means of enabling Speculative Store Bypass Disable. To allow simplified handling in virtual environments, hypervisors will expose an architectural definition through CPUID bit 0x80000008_EBX[25]. This needs to be exposed to guest OS running on AMD x86 hosts to allow them to protect against CVE-2018-3639.
Note that since this CPUID bit won't be present in the host CPUID results on physical hosts, it will not be enabled automatically in guests configured with "host-model" CPU unless using QEMU version >= 2.9.0. Thus for older versions of QEMU, this feature must be manually enabled using policy=force. Guests using the "host-passthrough" CPU mode do not need special handling.
[1] https://www.redhat.com/archives/libvir-list/2018-May/msg0156...