RE: [PATCH v2 0/3] VMSCAPE optimization for BHI variant

Kaplan, David
2025-10-16T15:58:05Z

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Sent: Wednesday, October 15, 2025 8:52 PM
> To: x86@kernel.org; H. Peter Anvin <hpa@zytor.com>; Josh Poimboeuf
> <jpoimboe@kernel.org>; Kaplan, David <David.Kaplan@amd.com>; Sean
> Christopherson <seanjc@google.com>; Paolo Bonzini <pbonzini@redhat.com>
> Cc: linux-kernel@vger.kernel.org; kvm@vger.kernel.org; Asit Mallick
> <asit.k.mallick@intel.com>; Tao Zhang <tao1.zhang@intel.com>
> Subject: [PATCH v2 0/3] VMSCAPE optimization for BHI variant
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> 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
>

Looks good to me.

Acked-by: David Kaplan <david.kaplan@amd.com>