An Interrupt Remapping Table (IRT) stores interrupt remapping configuration
for each device. In a normal operation, the AMD IOMMU caches the table
to optimize subsequent data accesses. This requires the IOMMU driver to
invalidate IRT whenever it updates the table. The invalidation process
includes issuing an INVALIDATE_INTERRUPT_TABLE command following by
a COMPLETION_WAIT command.

However, there are cases in which the IRT is updated at a high rate.
For example, for IOMMU AVIC, the IRTE[IsRun] bit is updated on every
vcpu scheduling (i.e. amd_iommu_update_ga()). On system with large
amount of vcpus and VFIO PCI pass-through devices, the invalidation
process could potentially become a performance bottleneck.

Introducing a new kernel boot option:

    amd_iommu=irtcachedis

which disables IRTE caching by setting the IRTCachedis bit in each IOMMU
Control register, and bypass the IRT invalidation process.
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Co-developed-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20230530141137.14376-4-suravee.suthikulpanit@amd.comSigned-off-by: Joerg Roedel <jroedel@suse.de>