AMD have updated their Spectre v2 guidance, and lfence/jmp is no longer
considered safe. AMD are recommending using retpoline everywhere.
Retpoline is incompatible with CET. All CET-capable hardware has efficient
IBRS (specifically, not something retrofitted in microcode), so use IBRS (and
STIBP for consistency sake).
This is a logical change on AMD, but not on Intel as the default calculations
would end up with these settings anyway. Leave behind a message if IBRS is
found to be missing.
Also update the default heuristics to never select THUNK_LFENCE. This causes
AMD CPUs to change their default to retpoline.
Also update the printed message to include the AMD MSR_SPEC_CTRL settings, and
STIBP now that we set it for consistency sake.
This is part of XSA-398 / CVE-2021-26401.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Andrew Cooper
8d03080d2a