[PATCH 0/3] KVM: arm64: nv: Fixes for stage-2 MMU recycling

[Oliver Upton <oliver.upton@linux.dev>]

I recently found that the path for reclaiming stage-2 MMUs is a bit
dodgy. We call kvm_stage2_unmap_range() from vcpu_load() when
repurposing a valid MMU, which itself is allowed to block. We repeat
this silliness on the next vcpu_load(), eventually leading to a stack
overflow.

It also looks like the nested hooks into the MMU notifiers are similarly
screwed up, ignoring the @may_block parameter and allowing the page
table walk to block unconditionally.

In addition to that, we are not handling vcpu->arch.hw_mmu safely in
preemptible code, i.e. we could be using a stale pointer on the loaded
hardware MMU.

Set of fixes to address all of the above, by:

1) Allowing vCPUs to 'pin' the loaded stage-2 MMU, guaranteeing that
   vcpu->arch.hw_mmu is stable when used in preemptible code

2) Preventing unmap walks from blocking when disallowed by the calling
   context.

   This is the correct fix for the MMU notifiers, and a "fix" for the
   reclaim case. Unmapping a whole MMU behind the write lock w/o periodically
   releasing the lock hurts vCPU scheduling, especially on larger VMs.

3) Push the cleanup of reclaimed MMUs into a vCPU request, allowing the
   walk to release the lock + CPU as needed.

Applies to 6.12-rc1, tested on M2. An easy way to force the reclaim case
is to constantly rerun a KVM selftest inside the L1 guest, which is how
I found this in the first place.

Oliver Upton (3):
  KVM: arm64: Treat stage-2 MMUs as refcounted generally
  KVM: arm64: nv: Do not block when unmapping stage-2 if disallowed
  KVM: arm64: nv: Punt stage-2 recycling to a vCPU request

 arch/arm64/include/asm/kvm_host.h   | 13 ++++++++--
 arch/arm64/include/asm/kvm_mmu.h    | 40 ++++++++++++++++++++++++++++-
 arch/arm64/include/asm/kvm_nested.h |  4 ++-
 arch/arm64/kvm/arm.c                | 12 +++++----
 arch/arm64/kvm/hyp/nvhe/hyp-main.c  |  2 +-
 arch/arm64/kvm/hyp/nvhe/pkvm.c      |  2 +-
 arch/arm64/kvm/hyp/nvhe/switch.c    |  2 +-
 arch/arm64/kvm/hyp/nvhe/tlb.c       |  4 +--
 arch/arm64/kvm/hyp/vhe/switch.c     |  2 +-
 arch/arm64/kvm/hyp/vhe/tlb.c        |  4 +--
 arch/arm64/kvm/mmu.c                | 34 +++++++++++++-----------
 arch/arm64/kvm/nested.c             | 39 ++++++++++++++++++++--------
 arch/arm64/kvm/sys_regs.c           |  6 ++---
 13 files changed, 119 insertions(+), 45 deletions(-)


base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
-- 
2.46.1.824.gd892dcdcdd-goog