[PATCH v5 0/8] Add SEV-SNP BTB Isolation and IBPB-on-Entry guest features

[Kim Phillips]

This series advertises two SEV-SNP guest-selectable side-channel
protections through KVM's vmsa_features ABI -- BTB Isolation
(SEV_FEATURES bit 7) and IBPB-on-Entry (SEV_FEATURES bit 21) -- along
with SVM_SEV_FEAT_SNP_ACTIVE, and fixes two x86 mitigation-selection
bugs that stand in the way.

The cpu/bugs patches come first because they are prerequisites, not
drive-by cleanups:

  - Patch 1 fixes spectre_v2=eibrs silently selecting retpolines instead
    of AutoIBRS on SNP hosts.  Commit acaa4b5c4c85 ("x86/speculation: Do
    not enable Automatic IBRS if SEV-SNP is enabled") suppressed AutoIBRS
    by never setting X86_FEATURE_IBRS_ENHANCED, which also disabled the
    explicit spectre_v2=eibrs request.  Set the feature bit and move the
    SNP-prefers-retpoline decision into the default/auto case, where it
    belongs.

  - Patch 2 allows spectre_v2=ibrs on non-Intel vendors.  BTB Isolation
    uses legacy SPEC_CTRL.IBRS toggling to optimize the SNP VM
    exit-to-re-entry path, so AMD needs to be able to select it.  It also
    makes EFER.AUTOIBRS explicitly track the selected mitigation, which
    matters across kexec since head_64.S preserves incoming EFER bits.

Patches 3-6 are small width/ABI cleanups and the SNP_ACTIVE and
SNP-only-feature-mask groundwork, and patches 7-8 add IBPB-on-Entry and
BTB Isolation themselves.

Note that BTB Isolation is advertised unconditionally on SNP-capable
parts.  That is deliberate: the APM defines it as architecturally
present on every SEV-SNP-capable processor, so unlike IBPB-on-Entry
there is no host enumeration bit to gate on.  Patch 8 spells this out.

Changes since v4:
  https://lore.kernel.org/all/20260804235611.4053375-1-kim.phillips@xxxxxxx/

 - Patch 1 ("x86/bugs: Allow forcing Automatic IBRS with SNP active
   using spectre_v2=eibrs"): only prefer retpoline over AutoIBRS on SNP
   hosts when CONFIG_MITIGATION_RETPOLINE is actually built in.  This
   folds v4's patch 3 into the same condition and avoids the
   select-then-fall-back ping pong.  Suggested in review:
   https://lore.kernel.org/all/lctyimdlenyb5kvfxarzajs2ggzkwn4yehxu233lojynqwa7ej@wjur3lh6szlr/

 - Patch 2 ("x86/bugs: Allow spectre_v2=ibrs on x86 vendors other than
   Intel"): move the EFER.AUTOIBRS kexec cleanup out of
   spectre_v2_apply_mitigation() and up to the top of
   cpu_select_mitigations(), next to the existing SPEC_CTRL kexec
   cleanup, so stale state is dropped before mitigation selection rather
   than during it.  spectre_v2_apply_mitigation() now only sets the bit,
   which keeps it much closer to its original shape.  Suggested in
   review:
   https://lore.kernel.org/all/ga63o6nf7xebndlo66qqasjx7axpopyph7ilkwhvx3enfaqs4w@r4qfr6eo6kcw/

 - Subject prefix fixes, per review:
   https://lore.kernel.org/all/20260818232204.GBaoTpHK8k7wD9mMk4@fat_crate.local/
   The two x86 patches go from "cpu/bugs:" to "x86/bugs:" to match the
   tip tree convention.  Patch 3 goes from "KVM: SEV:" to "KVM: SVM:",
   since it only touches arch/x86/include/asm/svm.h.

 - Tag cleanup, per review:
   https://lore.kernel.org/all/20260818232204.GBaoTpHK8k7wD9mMk4@fat_crate.local/
   Dropped Cc: stable@xxxxxxxxxx from both x86/bugs patches.  Patch 2
   drops its Fixes: too -- it enables spectre_v2=ibrs on a vendor that
   never had a use for the option rather than fixing a bug anyone is
   hitting, so neither tag was right.  Patch 1 drops the kernel test
   robot Reported-by:/Closes: pair; that report was against an earlier
   posting of this patch rather than against upstream, so there was
   nothing there for it to close.  The part of patch 1 that is genuinely
   -stable material -- an SNP host with CONFIG_MITIGATION_RETPOLINE=n
   booting with Spectre v2 unmitigated -- will be sent separately as a
   minimal backport once this lands.

 - Dropped v4's patch 3 ("cpu/bugs: Fall back to AutoIBRS when retpoline
   unavailable on SNP CPUs").  Its fallback is now subsumed by the
   CONFIG_MITIGATION_RETPOLINE test above.

 - Dropped v4's patch 1 ("x86/bugs: Only log missing retpoline when it's
   actually the missing mitigation").  After the rework above this
   series no longer needs it, but it still fixes a real spurious
   "no mitigation available!" on Intel + RETBleed with
   CONFIG_MITIGATION_IBRS_ENTRY=y, CONFIG_MITIGATION_RETPOLINE=n and
   spectre_v2=auto, where spectre_v2_update_mitigation() afterwards
   promotes SPECTRE_V2_NONE to SPECTRE_V2_IBRS.  Being posted separately
   as a standalone fix:
   https://lore.kernel.org/lkml/20260826222228.3668418-1-kim.phillips@xxxxxxx/

 - Added Tom's Reviewed-by to patch 8

Kim Phillips (8):
  x86/bugs: Allow forcing Automatic IBRS with SNP active using
    spectre_v2=eibrs
  x86/bugs: Allow spectre_v2=ibrs on x86 vendors other than Intel
  KVM: SVM: Define SVM_SEV_FEAT_* flags using BIT_ULL()
  KVM: selftests: sev_init2: Use BIT_ULL for VMSA feature bit definition
  KVM: SEV: Disallow setting SNP-only features for non-SNP guests via a
    single mask
  KVM: SEV: Advertise SVM_SEV_FEAT_SNP_ACTIVE
  KVM: SEV: Add support for IBPB-on-Entry
  KVM: SEV: Add support for SNP BTB Isolation

 arch/x86/Kconfig                              |  7 ++-
 arch/x86/include/asm/cpufeatures.h            |  1 +
 arch/x86/include/asm/svm.h                    | 17 ++++--
 arch/x86/kernel/cpu/bugs.c                    | 59 ++++++++++++++-----
 arch/x86/kernel/cpu/common.c                  |  6 +-
 arch/x86/kvm/svm/sev.c                        | 21 ++++++-
 tools/arch/x86/include/asm/cpufeatures.h      |  1 +
 .../selftests/kvm/x86/sev_init2_tests.c       | 20 +++++--
 8 files changed, 96 insertions(+), 36 deletions(-)


base-commit: ec8477a492cb24f2c334847c8734ca56c7ffdd29
-- 
2.43.0