linux-kernel - [PATCH v2 0/3] VMSCAPE optimization for BHI variant

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Message-ID: <20251015-vmscape-bhb-v2-0-91cbdd9c3a96@linux.intel.com>
Date: Wed, 15 Oct 2025 18:51:39 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	David Kaplan <david.kaplan@....com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Asit Mallick <asit.k.mallick@...el.com>,
	Tao Zhang <tao1.zhang@...el.com>
Subject: [PATCH v2 0/3] VMSCAPE optimization for BHI variant

v2:
- 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.

Roadmap:

- First patch introduces clear_bhb_long_loop() for processors with larger
  branch history tables.
- Second patch replaces IBPB on exit-to-userspace with branch history
  clearing sequence.

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

(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 (3):
      x86/bhi: Add BHB clearing for CPUs with larger branch history
      x86/vmscape: Replace IBPB with branch history clear on exit to userspace
      x86/vmscape: Remove LFENCE from BHB clearing long loop

 Documentation/admin-guide/hw-vuln/vmscape.rst   |  8 ++++
 Documentation/admin-guide/kernel-parameters.txt |  4 +-
 arch/x86/entry/entry_64.S                       | 63 ++++++++++++++++++-------
 arch/x86/include/asm/cpufeatures.h              |  1 +
 arch/x86/include/asm/entry-common.h             | 12 +++--
 arch/x86/include/asm/nospec-branch.h            |  5 +-
 arch/x86/kernel/cpu/bugs.c                      | 53 +++++++++++++++------
 arch/x86/kvm/x86.c                              |  5 +-
 8 files changed, 110 insertions(+), 41 deletions(-)
---
base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
change-id: 20250916-vmscape-bhb-d7d469977f2f

Best regards,
-- 
Pawan