Re: [PATCH v2 2/4] hw/riscv/virt-acpi-build: Add _DEP to ACPI0017 for CXL host bridge dependency

[Chen Pei <cp0613@linux.alibaba.com>]

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Hi Alistair,

On Mon, 6 Jul 2026 14:08:45 +1000, Alistair Francis wrote:
> What happens to kernels that don't have this change?

Good question. The behavior depends on how old the kernel is:

- Kernels that support _DEP processing but lack the
  acpi_dev_clear_dependencies() call in acpi_pci_root_add(): the
  ACPI0017 (CXLM) device will remain deferred indefinitely because
  its declared dependency on ACPI0016 is never cleared. CXL memory
  would be unusable on such kernels with this QEMU.

- Very old kernels that ignore _DEP entirely: the behavior is the
  same as v1 — the probe-ordering race between acpi_pci_root and
  cxl_acpi remains, but nothing is worse than before.

The kernel-side fix has already merged:
  https://lore.kernel.org/linux-pci/20260526025118.38935-1-cp0613@linux.alibaba.com/

So this is effectively a "new QEMU requires a sufficiently new kernel
for CXL to work" situation, similar to other QEMU features that depend
on kernel support.

Thanks,
Pei