[PATCH v4 0/8] KVM: x86: nSVM: Improve PAT virtualization

[Jim Mattson <jmattson@google.com>]

Currently, KVM's implementation of nested SVM treats the PAT MSR the same
way whether or not nested NPT is enabled: L1 and L2 share a single
PAT. However, the APM specifies that when nested NPT is enabled, the host
(L1) and the guest (L2) should have independent PATs: hPAT for L1 and gPAT
for L2. This patch series implements the architectural specification in
KVM.

Use the existing PAT MSR (vcpu->arch.pat) for hPAT. Add a new field,
svm->nested.gpat, for gPAT. With nested NPT enabled, redirect guest
accesses to the IA32_PAT MSR to gPAT. All other accesses, including
userspace accesses via KVM_{GET,SET}_MSRS, continue to reference hPAT.  The
special handling of userspace accesses ensures save/restore forward
compatibility (i.e. resuming a new checkpoint on an older kernel). When an
old kernel restores a checkpoint from a new kernel, the gPAT will be lost,
and L2 will simply use L1's PAT, which is the existing behavior of the old
kernel anyway.

v1: https://lore.kernel.org/kvm/20260113003016.3511895-1-jmattson@google.com/
v2: https://lore.kernel.org/kvm/20260115232154.3021475-1-jmattson@google.com/
v3: https://lore.kernel.org/kvm/20260205214326.1029278-1-jmattson@google.com/

  v3 -> v4:
   * Rebase on top of Yosry's v5 "Nested SVM fixes, cleanups, and hardening"
   * Rename the svm_set_vmcb_gpat() helper to vmcb_set_gpat() for
     consistency with other VMCB helpers [Yosry].
   * Cache g_pat within struct vmcb_save_area_cached (as
     svm->nested.save.g_pat) instead of using a standalone field in
     svm->nested [Sean].
   * Update nested_vmcb_check_save() to optionally validate the cached
     g_pat, depending on a new boolean argument [Yosry].
   * Reduce indentation in nested_vmcb02_prepare_save() when setting the
     guest PAT [Sean].
    

Jim Mattson (8):
  KVM: x86: nSVM: Clear VMCB_NPT clean bit when updating hPAT from guest
    mode
  KVM: x86: nSVM: Cache and validate vmcb12 g_pat
  KVM: x86: nSVM: Set vmcb02.g_pat correctly for nested NPT
  KVM: x86: nSVM: Redirect IA32_PAT accesses to either hPAT or gPAT
  KVM: x86: nSVM: Save gPAT to vmcb12.g_pat on VMEXIT
  KVM: x86: nSVM: Save/restore gPAT with KVM_{GET,SET}_NESTED_STATE
  KVM: x86: nSVM: Handle restore of legacy nested state
  KVM: selftests: nSVM: Add svm_nested_pat test

 arch/x86/include/uapi/asm/kvm.h               |   5 +
 arch/x86/kvm/svm/nested.c                     |  60 +++-
 arch/x86/kvm/svm/svm.c                        |  40 ++-
 arch/x86/kvm/svm/svm.h                        |  38 ++-
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../selftests/kvm/x86/svm_nested_pat_test.c   | 298 ++++++++++++++++++
 6 files changed, 413 insertions(+), 29 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86/svm_nested_pat_test.c

-- 
2.53.0.239.g8d8fc8a987-goog