To support accelerated SMMUv3 instances, introduce a shared system-wide
AddressSpace (shared_as_sysmem) that aliases the global system memory.
This shared AddressSpace will be used in a subsequent patch for all
vfio-pci devices behind all accelerated SMMUv3 instances within a VM.

Sharing a single system AddressSpace ensures that all devices behind
accelerated SMMUv3s use the same system address space pointer. This
allows VFIO/iommufd to reuse a single IOAS ID in iommufd_cdev_attach(),
enabling the Stage-2 page tables to be shared within the VM rather than
duplicated for each SMMUv3 instance.

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
---
 hw/arm/smmuv3-accel.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/hw/arm/smmuv3-accel.c b/hw/arm/smmuv3-accel.c
index 99ef0db8c4..b2eded743e 100644
--- a/hw/arm/smmuv3-accel.c
+++ b/hw/arm/smmuv3-accel.c
@@ -11,6 +11,14 @@
 #include "hw/arm/smmuv3.h"
 #include "smmuv3-accel.h"
 
+/*
+ * The root region aliases the global system memory, and shared_as_sysmem
+ * provides a shared Address Space referencing it. This Address Space is used
+ * by all vfio-pci devices behind all accelerated SMMUv3 instances within a VM.
+ */
+static MemoryRegion root, sysmem;
+static AddressSpace *shared_as_sysmem;
+
 static SMMUv3AccelDevice *smmuv3_accel_get_dev(SMMUState *bs, SMMUPciBus *sbus,
                                                PCIBus *bus, int devfn)
 {
@@ -51,9 +59,27 @@ static const PCIIOMMUOps smmuv3_accel_ops = {
     .get_address_space = smmuv3_accel_find_add_as,
 };
 
+static void smmuv3_accel_as_init(SMMUv3State *s)
+{
+
+    if (shared_as_sysmem) {
+        return;
+    }
+
+    memory_region_init(&root, OBJECT(s), "root", UINT64_MAX);
+    memory_region_init_alias(&sysmem, OBJECT(s), "smmuv3-accel-sysmem",
+                             get_system_memory(), 0,
+                             memory_region_size(get_system_memory()));
+    memory_region_add_subregion(&root, 0, &sysmem);
+
+    shared_as_sysmem = g_new0(AddressSpace, 1);
+    address_space_init(shared_as_sysmem, &root, "smmuv3-accel-as-sysmem");
+}
+
 void smmuv3_accel_init(SMMUv3State *s)
 {
     SMMUState *bs = ARM_SMMU(s);
 
     bs->iommu_ops = &smmuv3_accel_ops;
+    smmuv3_accel_as_init(s);
 }
-- 
2.43.0