On 11/20/25 2:21 PM, Shameer Kolothum wrote:
> 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>
> Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Eric
> ---
> 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);
> }