Re: [PATCH v3 0/1] KVM: arm64: vgic: Drop last_lr_irq and serialize overflow EOI replay

[Yuchao Zhang <ndaugoing@gmail.com>]

Hi Marc,

Thank you for the candid feedback and for cutting right to the heart of
the problem with the 3-patch series on your branch.

Your series is vastly simpler and elegant:
1. Only caching last_lr_irq when LRs are genuinely full eliminates the
   overhead for non-overflow exits.
2. Holding a proper reference on last_lr_irq and releasing it under
   ap_list_lock in vgic_prune_ap_list() cleanly addresses the lifetime
   hazard without introducing lock-order inversions during folding.
3. Halting the VM (kvm_arm_halt_guest) on remote GICR_CTLR.EnableLPIs=0
   completely avoids cross-vCPU concurrency for an event that almost
   never happens in practice.

As requested, I wrote a KVM arm64 selftest for this scenario:
tools/testing/selftests/kvm/arm64/vgic_lpi_disable.c

The test:
- Configures an ITS with multiple LPIs targeting vCPU 0.
- Keeps vCPU 0 in guest mode with EOImode=0 while injecting bursts of
  MSIs to force its List Registers to overflow into the ap_list.
- Concurrently, vCPU 1 repeatedly toggles GICR_CTLR.EnableLPIs on
  vCPU 0's redistributor.

Regarding testing: I currently no longer have access to KVM-capable
hardware (my test setup is no longer available), so I cannot provide
a Tested-by for your series. The race itself was originally identified
through code inspection rather than a standalone reproducer, which is
why I turned the theoretical scenario directly into this selftest.

The selftest builds cleanly as part of the full arm64 kvm selftest
suite, and its framework plumbing was smoke-tested under QEMU TCG,
which by construction cannot exercise the race itself. I'd be grateful
if you could give it a spin on your setup.

The selftest patch is based on top of your kvm-arm64/vgic-last_lr_irq-fixes
branch and has been sent in reply to this thread.

Thanks,
Yuchao