[PATCH v6 0/9] VMSCAPE optimization for BHI variant

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

v6:
- Remove semicolon at the end of asm in ALTERNATIVE (Uros).
- Fix build warning in vmscape_select_mitigation() (LKP).
- Rebased to v6.18.

v5: https://lore.kernel.org/r/20251126-vmscape-bhb-v5-2-02d66e423b00@linux.intel.com
- For BHI seq, limit runtime-patching to loop counts only (Dave).
  Dropped 2 patches that moved the BHB seq to a macro.
- Remove redundant switch cases in vmscape_select_mitigation() (Nikolay).
- Improve commit message (Nikolay).
- Collected tags.

v4: https://lore.kernel.org/r/20251119-vmscape-bhb-v4-0-1adad4e69ddc@linux.intel.com
- Move LFENCE to the callsite, out of clear_bhb_loop(). (Dave)
- Make clear_bhb_loop() work for larger BHB. (Dave)
  This now uses hardware enumeration to determine the BHB size to clear.
- Use write_ibpb() instead of indirect_branch_prediction_barrier() when
  IBPB is known to be available. (Dave)
- Use static_call() to simplify mitigation at exit-to-userspace. (Dave)
- Refactor vmscape_select_mitigation(). (Dave)
- Fix vmscape=on which was wrongly behaving as AUTO. (Dave)
- Split the patches. (Dave)
  - Patch 1-4 prepares for making the sequence flexible for VMSCAPE use.
  - Patch 5 trivial rename of variable.
  - Patch 6-8 prepares for deploying BHB mitigation for VMSCAPE.
  - Patch 9 deploys the mitigation.
  - Patch 10-11 fixes ON Vs AUTO mode.

v3: https://lore.kernel.org/r/20251027-vmscape-bhb-v3-0-5793c2534e93@linux.intel.com
- s/x86_pred_flush_pending/x86_predictor_flush_exit_to_user/ (Sean).
- Removed IBPB & BHB-clear mutual exclusion at exit-to-userspace.
- Collected tags.

v2: https://lore.kernel.org/r/20251015-vmscape-bhb-v2-0-91cbdd9c3a96@linux.intel.com
- Added check for IBPB feature in vmscape_select_mitigation(). (David)
- s/vmscape=auto/vmscape=on/ (David)
- Added patch to remove LFENCE from VMSCAPE BHB-clear sequence.
- Rebased to v6.18-rc1.

v1: https://lore.kernel.org/r/20250924-vmscape-bhb-v1-0-da51f0e1934d@linux.intel.com

Hi All,

These patches aim to improve the performance of a recent mitigation for
VMSCAPE[1] vulnerability. This improvement is relevant for BHI variant of
VMSCAPE that affect Alder Lake and newer processors.

The current mitigation approach uses IBPB on kvm-exit-to-userspace for all
affected range of CPUs. This is an overkill for CPUs that are only affected
by the BHI variant. On such CPUs clearing the branch history is sufficient
for VMSCAPE, and also more apt as the underlying issue is due to poisoned
branch history.

Below is the iPerf data for transfer between guest and host, comparing IBPB
and BHB-clear mitigation. BHB-clear shows performance improvement over IBPB
in most cases.

Platform: Emerald Rapids
Baseline: vmscape=off
Target: IBPB at VMexit-to-userspace Vs the new BHB-clear at
	VMexit-to-userspace mitigation (both compared against baseline).

(pN = N parallel connections)

| iPerf user-net | IBPB    | BHB Clear |
|----------------|---------|-----------|
| UDP 1-vCPU_p1  | -12.5%  |   1.3%    |
| TCP 1-vCPU_p1  | -10.4%  |  -1.5%    |
| TCP 1-vCPU_p1  | -7.5%   |  -3.0%    |
| UDP 4-vCPU_p16 | -3.7%   |  -3.7%    |
| TCP 4-vCPU_p4  | -2.9%   |  -1.4%    |
| UDP 4-vCPU_p4  | -0.6%   |   0.0%    |
| TCP 4-vCPU_p4  |  3.5%   |   0.0%    |

| iPerf bridge-net | IBPB    | BHB Clear |
|------------------|---------|-----------|
| UDP 1-vCPU_p1    | -9.4%   |  -0.4%    |
| TCP 1-vCPU_p1    | -3.9%   |  -0.5%    |
| UDP 4-vCPU_p16   | -2.2%   |  -3.8%    |
| TCP 4-vCPU_p4    | -1.0%   |  -1.0%    |
| TCP 4-vCPU_p4    |  0.5%   |   0.5%    |
| UDP 4-vCPU_p4    |  0.0%   |   0.9%    |
| TCP 1-vCPU_p1    |  0.0%   |   0.9%    |

| iPerf vhost-net | IBPB    | BHB Clear |
|-----------------|---------|-----------|
| UDP 1-vCPU_p1   | -4.3%   |   1.0%    |
| TCP 1-vCPU_p1   | -3.8%   |  -0.5%    |
| TCP 1-vCPU_p1   | -2.7%   |  -0.7%    |
| UDP 4-vCPU_p16  | -0.7%   |  -2.2%    |
| TCP 4-vCPU_p4   | -0.4%   |   0.8%    |
| UDP 4-vCPU_p4   |  0.4%   |  -0.7%    |
| TCP 4-vCPU_p4   |  0.0%   |   0.6%    |

[1] https://comsec.ethz.ch/research/microarch/vmscape-exposing-and-exploiting-incomplete-branch-predictor-isolation-in-cloud-environments/

---
Pawan Gupta (9):
      x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()
      x86/bhi: Make clear_bhb_loop() effective on newer CPUs
      x86/vmscape: Rename x86_ibpb_exit_to_user to x86_predictor_flush_exit_to_user
      x86/vmscape: Move mitigation selection to a switch()
      x86/vmscape: Use write_ibpb() instead of indirect_branch_prediction_barrier()
      x86/vmscape: Use static_call() for predictor flush
      x86/vmscape: Deploy BHB clearing mitigation
      x86/vmscape: Fix conflicting attack-vector controls with =force
      x86/vmscape: Add cmdline vmscape=on to override attack vector controls

 Documentation/admin-guide/hw-vuln/vmscape.rst   |  8 +++
 Documentation/admin-guide/kernel-parameters.txt |  4 +-
 arch/x86/Kconfig                                |  1 +
 arch/x86/entry/entry_64.S                       | 13 +++--
 arch/x86/include/asm/cpufeatures.h              |  2 +-
 arch/x86/include/asm/entry-common.h             |  9 ++--
 arch/x86/include/asm/nospec-branch.h            | 11 +++--
 arch/x86/kernel/cpu/bugs.c                      | 65 +++++++++++++++++++------
 arch/x86/kvm/x86.c                              |  4 +-
 arch/x86/net/bpf_jit_comp.c                     |  2 +
 10 files changed, 90 insertions(+), 29 deletions(-)
---
base-commit: 7d0a66e4bb9081d75c82ec4957c50034cb0ea449
change-id: 20250916-vmscape-bhb-d7d469977f2f

Best regards,
-- 
Thanks,
Pawan