[PATCH v4 0/3] KVM: PPC: Fixes for Book3S HV HPT locking and paired-single decoding

[Amit Machhiwal <amachhiw@linux.ibm.com>]

This series addresses bug fixes across KVM PPC Book3S HV
locking/synchronization and paired-single instruction decoding.

Patches 1 & 2 fix synchronization and preemption issues introduced in
commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for
HPT hcalls and page faults"):

- Patch 1 adds SRCU read lock protection when walking memslots during
  virtual-mode HPT hcalls to prevent use-after-free races with concurrent
  memslot updates.

- Patch 2 adds preempt_disable() around all virtual-mode HPTE bit-lock
  holders — both the guest vCPU paths (kvmppc_hpte_hv_fault() and
  kvmppc_pseries_do_hpt_hcall()) and the host-side paths
  (kvm_unmap_rmapp(), kvm_age_rmapp(), resize_hpt_rehash_hpte()) — to
  prevent CPU stalls and deadlocks on preemption.

Patch 3 fixes paired-single D-form instruction emulation:

- Patch 3 fixes get_d_signext() to correctly extract the full 12-bit D
  displacement field and perform proper two's-complement sign extension.

Testing:
========
All test kernel builds were compiled with CONFIG_DEBUG_ATOMIC_SLEEP=y.
The following scenarios were verified:

1. Power9 PowerNV (L0) in Radix mode:
   - Booted L0 host with kernel containing all 3 patches.
   - Booted KVM guests in both Radix and Hash modes.
   - Ran kernel build workload inside guests — no errors observed.
   - Booted L1 guest with the same kernel — booted and ran cleanly.

2. Power9 PowerNV (L0) in Hash mode:
   - Booted L0 host with kernel containing all 3 patches.
   - Booted KVM guest in Hash mode.
   - Ran kernel build workload inside guest — no errors observed.
   - Booted L1 nested guest with the same kernel — booted and ran cleanly.

3. Power10 LPAR (L1):
   - Booted Power10 LPAR with the patched kernel.
   - Booted KVM guest and ran workloads — no errors observed.

Changes in v4:
==============
- v3: https://lore.kernel.org/all/20261006122404.99358-1-amachhiw@linux.ibm.com/
- Patch 2:
  - Fixed missing preemption guard in kvm_htab_write(): introduced
    kvmppc_virtmode_do_h_remove(), a thin wrapper that brackets
    kvmppc_do_h_remove() with preempt_disable()/preempt_enable(), mirroring
    the existing kvmppc_virtmode_do_h_enter() pattern.  Replaced both raw
    kvmppc_do_h_remove() call sites in kvm_htab_write() with the new wrapper.
  - Updated commit message to document the kvm_htab_write() fix.
  - Dropped Reviewed-by from Shrikanth Hegde as the patch was materially
    extended.

Changes in v3:
==============
- v2: https://lore.kernel.org/all/20260930173750.56759-1-amachhiw@linux.ibm.com/
- Patch 2:
  - Fixed preemption window in kvm_unmap_rmapp() and kvm_age_rmapp(): moved
    preempt_disable() before lock_rmap() so both the rmap lock and the HPTE
    bit-lock are held under a single non-preemptible section, and added
    preempt_enable() on all early exits and retry paths before cpu_relax().
  - Cleaned up kvm_test_clear_dirty_npages(): removed redundant per-iteration
    preempt_disable()/preempt_enable() pairs since the caller
    kvmppc_hv_get_dirty_log_hpt() already holds preempt_disable() across the
    entire loop.
  - Reworded commit message to clearly detail the per-function locking design
    and rationale.
  - Picked up Reviewed-by tag from Shrikanth Hegde.
- Patch 3:
  - Picked up Reviewed-by tag from Shrikanth Hegde.

Changes in v2:
==============
- v1: https://lore.kernel.org/all/20260928122837.8782-1-amachhiw@linux.ibm.com/
- Patch 2: Extended preempt_disable()/preempt_enable() coverage to also wrap
  the HPTE bit-lock hold windows in four host-side virtual-mode functions:
  kvm_unmap_rmapp(), kvm_age_rmapp(), kvm_test_clear_dirty_npages(), and
  resize_hpt_rehash_hpte(). These were identified as vulnerable by Sashiko AI
  review and confirmed correct by audit.
- Dropped Reviewed-by from Ritesh as the patch was materially extended.
- Patch 2: Added warning comment above kvmppc_pseries_do_hpt_hcall()
  documenting the preemption requirement.

Amit Machhiwal (3):
  KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls
  KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE
    bit-lock users
  KVM: PPC: Fix get_d_signext() 12-bit displacement for paired-single
    D-form

 arch/powerpc/kvm/book3s_64_mmu_hv.c      | 33 +++++++++-
 arch/powerpc/kvm/book3s_hv.c             | 80 +++++++++++++-----------
 arch/powerpc/kvm/book3s_paired_singles.c |  7 +--
 3 files changed, 77 insertions(+), 43 deletions(-)


base-commit: 602042bf29f6efde39cfb5fdd9289bf4854bc0c5
-- 
2.54.0 (Apple Git-157)