New iotcls were introduced to pass information about guest stage1 to the host through VFIO. Let's document the nested stage control. Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx> --- v2 -> v3: - document the new fault API v1 -> v2: - use the new ioctl names - add doc related to fault handling --- Documentation/vfio.txt | 83 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 83 insertions(+) diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt index f1a4d3c3ba0b..aab59ddf5ebd 100644 --- a/Documentation/vfio.txt +++ b/Documentation/vfio.txt @@ -239,6 +239,89 @@ group and can access them as follows:: /* Gratuitous device reset and go... */ ioctl(device, VFIO_DEVICE_RESET); +IOMMU Dual Stage Control +------------------------ + +Some IOMMUs support 2 stages/levels of translation. "Stage" corresponds to +the ARM terminology while "level" corresponds to Intel's VTD terminology. In +the following text we use either without distinction. + +This is useful when the guest is exposed with a virtual IOMMU and some +devices are assigned to the guest through VFIO. Then the guest OS can use +stage 1 (IOVA -> GPA), while the hypervisor uses stage 2 for VM isolation +(GPA -> HPA). + +The guest gets ownership of the stage 1 page tables and also owns stage 1 +configuration structures. The hypervisor owns the root configuration structure +(for security reason), including stage 2 configuration. This works as long +configuration structures and page table format are compatible between the +virtual IOMMU and the physical IOMMU. + +Assuming the HW supports it, this nested mode is selected by choosing the +VFIO_TYPE1_NESTING_IOMMU type through: + +ioctl(container, VFIO_SET_IOMMU, VFIO_TYPE1_NESTING_IOMMU); + +This forces the hypervisor to use the stage 2, leaving stage 1 available for +guest usage. + +Once groups are attached to the container, the guest stage 1 translation +configuration data can be passed to VFIO by using + +ioctl(container, VFIO_IOMMU_BIND_PASID_TABLE, &pasid_table_info); + +This allows to combine guest stage 1 configuration structure along with +hypervisor stage 2 configuration structure. stage 1 configuration structures +are dependent on the IOMMU type. + +As the stage 1 translation is fully delegated to the HW, physical events that +may occur (especially translation faults), need to be propagated up to +the virtualizer and re-injected into the guest. + +The userspace must be prepared to receive faults. The VFIO-PCI device +exposes 2 regions dedicated to HW faults: one read-only "producer" fault +region (kernel is the producer and writes into this region) and one +write-only "consumer" fault region, type/subtype respectively: +- VFIO_REGION_TYPE_NESTED/VFIO_REGION_SUBTYPE_NESTED_FAULT_PROD +- VFIO_REGION_TYPE_NESTED/VFIO_REGION_SUBTYPE_NESTED_FAULT_CONS + +The producer fault region exposes a VFIO_REGION_INFO_CAP_PRODUCER_FAULT +region capability that allows the userspace to retrieve the max fault +ABI version supported by the kernel. + +The ABI version can be negotiated: the userspace writes the version it +wants in the consumer region (greater or equal than 1). Once set, the +ABI version cannot be changed. + +Then by using VFIO_DEVICE_SET_IRQS along with the VFIO_PCI_DMA_FAULT_IRQ_INDEX +index, the virtualizer can register an eventfd signalled whenever a fault is +observed at physical level. + +The kernel writes the fault records formatted according to the negotiated +ABI version in the producer region fault queue. This part of the producer +fault region can be mmapped (see VFIO_REGION_INFO_CAP_SPARSE_MMAP result). + +When the userspace consumes a fault in the queue, it should increment +the consumer index to allow new fault records to replace the used ones. +The queue size and the entry size can be retrieved in the producer region. +The consumer index should never overshoot the producer index as in any +other circular buffer scheme. Also it must be less than the queue size +otherwise the change is ignored by the kernel. + +When the guest invalidates stage 1 related caches, invalidations must be +forwarded to the host through +ioctl(container, VFIO_IOMMU_CACHE_INVALIDATE, &inv_data); +Those invalidations can happen at various granularity levels, page, context, ... + +The ARM SMMU specification introduces another challenge: MSIs are translated by +both the virtual SMMU and the physical SMMU. To build a nested mapping for the +IOVA programmed into the assigned device, the guest needs to pass its IOVA/MSI +doorbell GPA binding to the host. Then the hypervisor can build a nested stage 2 +binding eventually translating into the physical MSI doorbell. + +This is achieved by +ioctl(container, VFIO_IOMMU_BIND_MSI, &guest_binding); + VFIO User API ------------------------------------------------------------------------------- -- 2.20.1
