x86/iommu: Bootup stage cleanups

Boris reported that the conversion to stacked irqdomains causes a GFP_KERNEL allocation from atomic context splat.

The callchain is:

kernel_init_freeable
smp_prepare_cpus
default_setup_apic_routing
enable_IR_x2apic
irq_remapping_enable
intel_enable_irq_remapping
intel_setup_irq_remapping
irq_domain_add_hierarchy
__irq_domain_add

The reason for the splat is that smp_prepare_cpus() is pointlessly disabling preemption and enable_IR_x2apic is calling irq_remapping_enable with interrupts disabled.

Removing the completely useless preemption disable from smp_prepare_cpus() is a no brainer, but resolving the interrupt disabled issue in enable_IR_x2apic/irq_remapping_enable forced me to look deeper into the iommu setup code.

Convoluted is probably an euphemism for that maze.

The early detection of iommus happens in mem_init() which looks for the availability of ACPI tables describing iommus.

The actual parsing of the tables happens for AMD in setup_irq_remapping_ops() which calls remap_ops.prepare() and for Intel in irq_remapping_enable().

The AMD parsing allocates data structures, memory pages and the irq domain from the prepare callback, which makes a lot of sense as this has nothing to do with the actual enablement. That AMD code uses GFP_KERNEL allocations inside of a preempt disabled region. That issue is solved by removing the silly preempt_disable() from smp_prepare_cpus().

For Intel the actual parsing and allocation is moved from the enable to the prepare callback. That cures the reported splat and allows to use GFP_KERNEL allocations for the iommu internal stuff as well. While in the early bootup it does not matter much, for physical hotplug GFP_ATOMIC allocations are not desired at all.

That solves the issue at hand, but the iommu setup stays a tangled maze with completely non obvious setup mechanisms depending on the options (iommu on/off - irq remapping on/off).

Thanks,

tglx

---
 arch/x86/include/asm/irq_remapping.h |    2 -
 arch/x86/kernel/apic/apic.c          |    3 -
 arch/x86/kernel/smpboot.c            |    8 ----
 drivers/iommu/intel_irq_remapping.c  |   68 +++++++++++++++++++++++------------
 drivers/iommu/irq_remapping.c        |   19 +++------
 kernel/irq/irqdomain.c               |   21 +---------
 6 files changed, 56 insertions(+), 65 deletions(-)