x86/spec-ctrl: Mitigation Register File Data Sampling

x86/spec-ctrl: Mitigation Register File Data Sampling

RFDS affects Atom cores, also branded E-cores, between the Goldmont and
Gracemont microarchitectures.  This includes Alder Lake and Raptor Lake hybrid
clien systems which have a mix of Gracemont and other types of cores.

Two new bits have been defined; RFDS_CLEAR to indicate VERW has more side
effets, and RFDS_NO to incidate that the system is unaffected.  Plenty of
unaffected CPUs won't be getting RFDS_NO retrofitted in microcode, so we
synthesise it.  Alder Lake and Raptor Lake Xeon-E's are unaffected due to
their platform configuration, and we must use the Hybrid CPUID bit to
distinguish them from their non-Xeon counterparts.

Like MD_CLEAR and FB_CLEAR, RFDS_CLEAR needs OR-ing across a resource pool, so
set it in the max policies and reflect the host setting in default.

This is part of XSA-452 / CVE-2023-28746.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit fb5b6f6744713410c74cfc12b7176c108e3c9a31)