[PATCH 0/2] iommu/amd: Preserve x2APIC and MSI support when amd_iommu=off

[Adrian Huang (Lenovo)]

[Overview]
This patch series modifies the behavior of the amd_iommu=off kernel
parameter to ensure that x2APIC and MSI domain allocation remain
functional, even when DMA translation is disabled.

Currently, on AMD systems, specifying amd_iommu=off completely shuts
down the IOMMU subsystem. This causes the system to disable x2APIC and
fall back to physical flat mode, which limits the system to 255 APIC
IDs. On high-core-count systems (e.g., 448 cores), this results in a
significant number of CPUs failing to bring up. Furthermore, dynamically
added PCI devices (such as SR-IOV VFs) fail to associate with the
correct MSI domain, leading to IOMMU fault events and kernel warnings.

[Motivation]
On Intel platforms, intel_iommu=off only disables DMA translation while
keeping the necessary interrupt remapping infrastructure active to
support x2APIC and high CPU counts. This series brings the AMD IOMMU
driver in line with this behavior, ensuring stability on modern,
large-scale hardware.

[Patch Summary]
Patch 1: Adjusts the amd_iommu=off semantics to keep x2APIC enabled. It
ensures that the kernel can bring up all available CPUs on large systems
by maintaining x2APIC ops instead of falling back to physical flat mode.

Patch 2: With applying "Patch 1", patch 2 ensures the PCI bus notifier
is registered even when amd_iommu=off is used. This allows hotplugged
devices or SR-IOV VFs to be correctly assigned to the IOMMU interrupt
remapping domain, preventing INVALID_DEVICE_REQUEST faults and MSI
activation failures.

[Verification]
Testing was performed on a 448-core Lenovo ThinkSystem server. All 448
CPUs are successfully initialized; SR-IOV VFs are created and assigned
interrupts without faults.

Adrian Huang (Lenovo) (2):
  iommu/amd: Keep x2apic enabled when appending amd_iommu=off
  iommu/amd: Register PCI bus notifier when appending amd_iommu=off

 drivers/iommu/amd/amd_iommu.h |  1 +
 drivers/iommu/amd/init.c      | 81 ++++++++++++++++++++++++++++++-----
 drivers/iommu/amd/iommu.c     | 14 ++++++
 3 files changed, 86 insertions(+), 10 deletions(-)

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2.47.3