iommu/vt-d: Introduce a rwsem to protect global data structures

Introduce a global rwsem dmar_global_lock, which will be used to
protect DMAR related global data structures from DMAR/PCI/memory
device hotplug operations in process context.

DMA and interrupt remapping related data structures are read most,
and only change when memory/PCI/DMAR hotplug event happens.
So a global rwsem solution is adopted for balance between simplicity
and performance.

For interrupt remapping driver, function intel_irq_remapping_supported(),
dmar_table_init(), intel_enable_irq_remapping(), disable_irq_remapping(),
reenable_irq_remapping() and enable_drhd_fault_handling() etc
are called during booting, suspending and resuming with interrupt
disabled, so no need to take the global lock.

For interrupt remapping entry allocation, the locking model is:
	down_read(&dmar_global_lock);
	/* Find corresponding iommu */
	iommu = map_hpet_to_ir(id);
	if (iommu)
		/*
		 * Allocate remapping entry and mark entry busy,
		 * the IOMMU won't be hot-removed until the
		 * allocated entry has been released.
		 */
		index = alloc_irte(iommu, irq, 1);
	up_read(&dmar_global_lock);

For DMA remmaping driver, we only uses the dmar_global_lock rwsem to
protect functions which are only called in process context. For any
function which may be called in interrupt context, we will use RCU
to protect them in following patches.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>

1 files changed, 16 insertions, 3 deletions

diff --git a/drivers/iommu/dmar.c b/drivers/iommu/dmar.c
index 4ae6df27ad5d..c9aca8841fa0 100644
--- a/drivers/iommu/dmar.c
+++ b/drivers/iommu/dmar.c
@@ -43,10 +43,19 @@
 
 #include "irq_remapping.h"
 
-/* No locks are needed as DMA remapping hardware unit
- * list is constructed at boot time and hotplug of
- * these units are not supported by the architecture.
+/*
+ * Assumptions:
+ * 1) The hotplug framework guarentees that DMAR unit will be hot-added
+ *    before IO devices managed by that unit.
+ * 2) The hotplug framework guarantees that DMAR unit will be hot-removed
+ *    after IO devices managed by that unit.
+ * 3) Hotplug events are rare.
+ *
+ * Locking rules for DMA and interrupt remapping related global data structures:
+ * 1) Use dmar_global_lock in process context
+ * 2) Use RCU in interrupt context
  */
+DECLARE_RWSEM(dmar_global_lock);
 LIST_HEAD(dmar_drhd_units);
 
 struct acpi_table_header * __initdata dmar_tbl;
@@ -565,6 +574,7 @@ int __init detect_intel_iommu(void)
 {
 	int ret;
 
+	down_write(&dmar_global_lock);
 	ret = dmar_table_detect();
 	if (ret)
 		ret = check_zero_address();
@@ -582,6 +592,7 @@ int __init detect_intel_iommu(void)
 	}
 	early_acpi_os_unmap_memory((void __iomem *)dmar_tbl, dmar_tbl_size);
 	dmar_tbl = NULL;
+	up_write(&dmar_global_lock);
 
 	return ret ? 1 : -ENODEV;
 }
@@ -1394,10 +1405,12 @@ static int __init dmar_free_unused_resources(void)
 	if (irq_remapping_enabled || intel_iommu_enabled)
 		return 0;
 
+	down_write(&dmar_global_lock);
 	list_for_each_entry_safe(dmaru, dmaru_n, &dmar_drhd_units, list) {
 		list_del(&dmaru->list);
 		dmar_free_drhd(dmaru);
 	}
+	up_write(&dmar_global_lock);
 
 	return 0;
 }