[v2,0/6] IBPB cleanups and a fixup

Message

Yosry Ahmed Feb. 27, 2025, 1:27 a.m. UTC

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.

We should also unify indirect_branch_prediction_barrier() with
entry_ibpb() to have a single IBPB primitive that always stuffs the RSB
if needed. Josh will be sending follow up changes for that.

v1 -> v2:
- Collected Ack/Review tags (thanks everyone!).
- Combined the if statements in vmx_vcpu_load_vmcs() and moved the
  static branch check ahead (Sean Christopherson).
- Renamed the static branch to switch_vcpu_ibpb (Sean Christopherson).
- Fixed a typo in the commit message (Jim Mattson).

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 switch
  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               | 6 +++++-
 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(+), 8 deletions(-)