iommu/amd: Disallow implicit START_STATE => IVRS_DETECTED transition

Xen PV intentionally skips calling amd_iommu_detect() in order to gate
IOMMU initialization, as the Xen hypervisor has taken over the hardware.
However, due to amd_iommu_prepare() requesting ACPI_FINISHED while
setting up APIC, the state machine is still progressed, only to be
stopped midway due to the `!amd_iommu_detected' check in
early_amd_iommu_init(). This is worthless and fragile. It unnecessarily
calls detect_ivrs(), which leads to extra overhead, i.e., getting the
IVRS table and accessing the PCI config space.

Disallow implicit START_STATE => IVRS_DETECTED transition, so that
amd_iommu_prepare() and other paths no longer progress the state machine
accidentally. This should also help prevent potential bugs if more
housekeeping work is added to amd_iommu_detect() in the future.

Signed-off-by: Rong Zhang <i@rong.moe>
Reviewed-by: Ankit Soni <Ankit.Soni@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

0 files changed, 0 insertions, 0 deletions