KVM: arm64: Add documentation for pKVM DMA isolation

Populate the section for DMA isolation in pKVM with the newly
added KVM IOMMU and pKVM SMMUv3 driver details.

Signed-off-by: Mostafa Saleh <smostafa@google.com>

Documentation/virt/kvm/arm/pkvm.rst

diff --git a/Documentation/virt/kvm/arm/pkvm.rst b/Documentation/virt/kvm/arm/pkvm.rst
index 514992a..ce67e2a 100644
--- a/Documentation/virt/kvm/arm/pkvm.rst
+++ b/Documentation/virt/kvm/arm/pkvm.rst
@@ -77,7 +77,28 @@
 DMA isolation using an IOMMU
 ----------------------------
 
-Status: **Unimplemented.**
+Status: Supported for devices behind SMMUv3 supporting dual stages
+of translation.
+
+With ``CONFIG_ARM_SMMU_V3_PKVM``, the hypervisor will take over the SMMUs
+on the system and provide an architectural emulation to the kernel SMMUv3
+driver.
+
+If some devices are not behind an IOMMU or behind another IOMMU architecture,
+DMA isolation is not supported, as a driver must be provided for that.
+
+DMA isolation is enforced by dual stages of translation; similar to the CPU
+where a driver can register their ops through ``kvm_iommu_register_driver``
+and implement ``host_stage2_idmap`` to shadow the CPU page table.
+
+The page tables for the stage-2 SMMUv3 will be allocated from a pool created
+at boot with size configured from the command line "kvm-arm.iommu_pgt_mem"
+The size must cover the worst case scenario for leaf mappings of system memory.
+
+This implementation trusts the system firmware not to allow the untrusted
+host kernel to bypass the SMMUv3.
+For example, by resetting the power. In that case, it is the firmware's
+responsibility to save/restore the SMMUv3 state.
 
 Proxying of Trustzone services
 ------------------------------