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

Pawan Gupta
2025-10-16T17:23:08Z

On Thu, Oct 16, 2025 at 03:57:56PM +0000, Kaplan, David wrote:
> [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>

Thanks.