[PATCH 0/6] IBPB cleanups and a fixup

Yosry Ahmed
2025-02-19T22:08:38Z

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.

I wanted to do more, but decided to stay conservative. I was mainly
hoping to merge indirect_branch_prediction_barrier() with entry_ibpb()
to have a single IBPB primitive that always stuffs the RSB if the IBPB
doesn't, but this would add some overhead in paths that currently use
indirect_branch_prediction_barrier(), and I was not sure if that's
acceptable.

For the record, my measurements of the latency of
indirect_branch_prediction_barrier() and entry_ibpb() on Rome and Milan
(both do not have X86_FEATURE_AMD_IBPB_RET) are as follows:

Rome:
400ns (indirect_branch_prediction_barrier) vs 500ns (entry_ibpb)

Milan:
220ns (indirect_branch_prediction_barrier) vs 280ns (entry_ibpb)

I also wanted to move controlling the IBPB on vCPU load from
being under spectre_v2_user to spectre_v2, because "user" in a lot of
mitigation contexts does not include VMs.

Just laying out these thoughts in case others have any comments.

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 load
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 | 7 +++++--
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(+), 9 deletions(-)

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2.48.1.601.g30ceb7b040-goog