[PATCH v3 0/4] KVM: arm64: vgic: Fix IGROUPR writability and IIDR revision control

[David Woodhouse <dwmw2@infradead.org>]

Maintaining precise guest compatibility across host kernel upgrades — 
and even downgrades, since rollback is sometimes necessary — is not 
optional. That *shouldn't* need saying, but maybe it does:
https://lore.kernel.org/all/6856b269d2af706eae397e0cf9c1231f89d9a932.camel@infradead.org/

This series fixes the GICv2/v3 IGROUPR writability model to be
consistently controlled by the GICD_IIDR implementation revision,
replacing the ad-hoc v2_groups_user_writable flag.
  
Before commit d53c2c29ae0d ("KVM: arm/arm64: vgic: Allow configuration
of interrupt groups"), IGROUPR was read-only on both GICv2 and GICv3.
That commit made it unconditionally guest-writable, but provided no
mechanism for userspace to restore the previous behaviour for guests
that were launched on older kernels (which which might need to be
migrated back to older kernels).

The v2_groups_user_writable flag (added by 32f8777ed92d) attempted to
gate GICv2 userspace IGROUPR writes until userspace wrote the IIDR,
but the guest write path was never gated, creating an inconsistency
where the guest could modify groups that userspace couldn't save or
restore.  QEMU never writes GICD_IIDR, so its GICv2 IGROUPR
save/restore (QEMU commit eb8b9530b0c) appears to be silently broken.
  
This series:
 - Allows userspace to set IIDR revision 1, to restore the original
   read-only IGROUPR behaviour for both GICv2 and GICv3 (patch 1)
 - Removes v2_groups_user_writable and makes both guest and userspace
   writability follow the IIDR revision directly (patch 3)
 - Adds selftests covering IIDR revision semantics and a QEMU-style
   save/restore scenario (patches 2, 4)
  
Tested on EC2 c7g.metal (GICv3 native) and under QEMU-TCG (GICv2).

David Woodhouse (4):
      KVM: arm64: vgic: Allow userspace to set IIDR revision 1
      KVM: arm64: selftests: Add vgic IIDR revision test
      KVM: arm64: vgic: Remove v2_groups_user_writable and use IIDR revision directly
      KVM: arm64: selftests: Add GICv2 IGROUPR writability test

 arch/arm64/kvm/vgic/vgic-mmio-v2.c                 |  15 +-
 arch/arm64/kvm/vgic/vgic-mmio-v3.c                 |   4 +
 arch/arm64/kvm/vgic/vgic-mmio.c                    |   4 +
 include/kvm/arm_vgic.h                             |   4 +-
 tools/testing/selftests/kvm/Makefile.kvm           |   2 +
 .../testing/selftests/kvm/arm64/vgic_group_iidr.c  | 118 +++++++++++
 tools/testing/selftests/kvm/arm64/vgic_group_v2.c  | 226 +++++++++++++++++++++
 7 files changed, 361 insertions(+), 12 deletions(-)