This series removes X86_FEATURE_USE_IBPB, and fixes a KVM nVMX bug in the process. The motivation is mostly the confusing name of X86_FEATURE_USE_IBPB, which sounds like it controls IBPBs in general, but it only controls IBPBs for spectre_v2_mitigation. A side effect of this confusion is the nVMX bug, where virtualizing IBRS correctly depends on the spectre_v2_user mitigation. The feature bit is mostly redundant, except in controlling the IBPB in the vCPU load path. For that, a separate static branch is introduced, similar to switch_mm_*_ibpb. I wanted to do more, but decided to stay conservative. I was mainly hoping to merge indirect_branch_prediction_barrier() with entry_ibpb() to have a single IBPB primitive that always stuffs the RSB if the IBPB doesn't, but this would add some overhead in paths that currently use indirect_branch_prediction_barrier(), and I was not sure if that's acceptable. For the record, my measurements of the latency of indirect_branch_prediction_barrier() and entry_ibpb() on Rome and Milan (both do not have X86_FEATURE_AMD_IBPB_RET) are as follows: Rome: 400ns (indirect_branch_prediction_barrier) vs 500ns (entry_ibpb) Milan: 220ns (indirect_branch_prediction_barrier) vs 280ns (entry_ibpb) I also wanted to move controlling the IBPB on vCPU load from being under spectre_v2_user to spectre_v2, because "user" in a lot of mitigation contexts does not include VMs. Just laying out these thoughts in case others have any comments. Yosry Ahmed (6): x86/bugs: Move the X86_FEATURE_USE_IBPB check into callers x86/mm: Remove X86_FEATURE_USE_IBPB checks in cond_mitigation() x86/bugs: Remove the X86_FEATURE_USE_IBPB check in ib_prctl_set() x86/bugs: Use a static branch to guard IBPB on vCPU load KVM: nVMX: Always use IBPB to properly virtualize IBRS x86/bugs: Remove X86_FEATURE_USE_IBPB arch/x86/include/asm/cpufeatures.h | 1 - arch/x86/include/asm/nospec-branch.h | 4 +++- arch/x86/kernel/cpu/bugs.c | 7 +++++-- arch/x86/kvm/svm/svm.c | 3 ++- arch/x86/kvm/vmx/vmx.c | 3 ++- arch/x86/mm/tlb.c | 3 +-- tools/arch/x86/include/asm/cpufeatures.h | 1 - 7 files changed, 13 insertions(+), 9 deletions(-) -- 2.48.1.601.g30ceb7b040-goog
