[PATCH 0/3] Unify VERW mitigation for guests

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

This series unifies the VERW execution sites in KVM, specifically
addressing inconsistencies in how MMIO Stale Data mitigation is handled
compared to other data sampling attacks (MDS/TAA/RFDS).

Problem
=======
Currently, MMIO Stale Data mitigation is handled differently from other
VERW-based mitigations. While MDS/TAA/RFDS perform VERW clearing in
assembly code, MMIO Stale Data mitigation uses a separate code path with
x86_clear_cpu_buffer() calls. This inconsistency exists because MMIO Stale
Data mitigation only needs to be applied when guests can access host MMIO,
which was previously difficult to check in assembly. The other
inconsistency is VERW execution MMIO Stale Data dependency on L1TF
mitigation.

Solution
========
Remove the VERW mitigation for MMIO in C, and use the asm VERW callsite for
all VERW mitigations in KVM. Also decoupling MMIO mitigation from L1TF
mitigation.

Roadmap:

Patch 1: Switch to VM_CLEAR_CPU_BUFFERS usage in VMX to align Intel
	 mitigations with AMD's TSA mitigation.

Patch 2: Renames cpu_buf_vm_clear to cpu_buf_vm_clear_mmio_only to
	 avoid confusion with the broader X86_FEATURE_CLEAR_CPU_BUF_VM.

Patch 3: Unifies MMIO Stale Data mitigation with other VERW-based
         mitigations.

---
Pawan Gupta (3):
      x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well
      x86/mmio: Rename cpu_buf_vm_clear to cpu_buf_vm_clear_mmio_only
      x86/mmio: Unify VERW mitigation for guests

 arch/x86/include/asm/nospec-branch.h |  2 +-
 arch/x86/kernel/cpu/bugs.c           | 17 +++++++++++------
 arch/x86/kvm/mmu/spte.c              |  2 +-
 arch/x86/kvm/vmx/run_flags.h         | 12 ++++++------
 arch/x86/kvm/vmx/vmenter.S           |  8 +++++++-
 arch/x86/kvm/vmx/vmx.c               | 26 ++++++++++----------------
 6 files changed, 36 insertions(+), 31 deletions(-)
---
base-commit: dcb6fa37fd7bc9c3d2b066329b0d27dedf8becaa
change-id: 20251028-verw-vm-6b922910a2b3

Best regards,
-- 
Pawan