Messages in this thread | | | Date | Wed, 23 Aug 2023 07:57:20 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH 05/22] x86/srso: Fix SBPB enablement for mitigations=off |
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On Sun, Aug 20, 2023 at 06:19:02PM -0700, Josh Poimboeuf wrote: > If the user has requested no mitigations with mitigations=off, use the > lighter-weight SBPB instead of IBPB for other mitigations. > > Note that even with mitigations=off, IBPB/SBPB may still be used for > Spectre v2 user <-> user protection. Whether that makes sense is a > question for another day.
Well, with my user hat on, off means off.
IINM, spectre_v2_parse_cmdline() will give SPECTRE_V2_CMD_NONE to spectre_v2_select_mitigation() when mitigations=off.
spectre_v2_user_select_mitigation() will use the spectre_v2_select_mitigation()'s result, which turn into SPECTRE_V2_USER_CMD_NONE and then not enable *BPB either.
So even if we set x86_pred_cmd to SBPB here, it won't do anything because X86_FEATURE_USE_IBPB won't be set and indirect_branch_prediction_barrier() will be a NOP.
IOW, I think we should separate the check:
if (cpu_mitigations_off()) return;
at the beginning of srso_select_mitigation() so that it is crystal clear. Maybe even slap a comment over it.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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