[v4,0/3] Distinguish between variants of IBPB

Message

Jim Mattson Sept. 13, 2024, 5:32 p.m. UTC

Prior to Zen4, AMD's IBPB did not flush the RAS (or, in Intel
terminology, the RSB). Hence, the older version of AMD's IBPB was not
equivalent to Intel's IBPB. However, KVM has been treating them as
equivalent, synthesizing Intel's CPUID.(EAX=7,ECX=0):EDX[bit 26] on any
platform that supports the synthetic features X86_FEATURE_IBPB and
X86_FEATURE_IBRS.

Equivalence also requires a previously ignored feature on the AMD side,
CPUID Fn8000_0008_EBX[IBPB_RET], which is enumerated on Zen4.

v4: Added "guaranteed" to X86_FEATURE_IBPB comment [Pawan]
    Changed logic for deducing AMD IBPB features from Intel IBPB features
    in kvm_set_cpu_caps [Tom]
    Intel CPUs that suffer from PBRSB can't claim AMD_IBPB_RET [myself]

v3: Pass through IBPB_RET from hardware to userspace. [Tom]
    Derive AMD_IBPB from X86_FEATURE_SPEC_CTRL rather than
    X86_FEATURE_IBPB. [Tom]
    Clarify semantics of X86_FEATURE_IBPB.

v2: Use IBPB_RET to identify semantic equality. [Venkatesh]

Jim Mattson (3):
  x86/cpufeatures: Define X86_FEATURE_AMD_IBPB_RET
  KVM: x86: Advertise AMD_IBPB_RET to userspace
  KVM: x86: AMD's IBPB is not equivalent to Intel's IBPB

 arch/x86/include/asm/cpufeatures.h |  3 ++-
 arch/x86/kvm/cpuid.c               | 12 +++++++++---
 2 files changed, 11 insertions(+), 4 deletions(-)