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In particular, for zen2 the options > *might* include ibpb or unret. > > But I have failed to find what 'unret' actually means. Any > pointers, please ? > > While ibpb might be available (and slow), on my Renoir with > microcode level (0860106h) there were no newer microcode versions > available when I last looked (a few weeks ago) but note 7 at the > bottom of > https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v > 7_20220712.pdf > implies that the relevant bit is only set on Renoir in 0860109h and > later. > > Some of the text in that pdf implies that at least one of the > options could be set if not already set from the microcode, but the > amount of detail leaves me totally lost. > > Assuming, for the moment, that I might want to try this full > mitigation, is there any way to set this in the absence of newer > microcode ? > > Or should I just accept that the best I can get is 'unret', whatever > that means ? > > ĸen 'unret' = AMD JMP2RET i.e. replace all 'ret' instructions with 'jmp __x86_return_thunk', and safe training the thunk code upon kernel/hypervisor entry. This is a purely software mitigation, it doesn't require any microcode. AMD JMP2RET is described in this document: https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v7_20220712.pdf alex.