On Sun, Sep 19, 2021 at 02:38:48PM +0800, Liu Yi L wrote: > Document the /dev/iommu framework for user. > > Open: > Do we want to document /dev/iommu in Documentation/userspace-api/iommu.rst? > Existing iommu.rst is for the vSVA interfaces, honestly, may need to rewrite > this doc entirely. > > Signed-off-by: Kevin Tian > Signed-off-by: Liu Yi L > --- > Documentation/userspace-api/index.rst | 1 + > Documentation/userspace-api/iommufd.rst | 183 ++++++++++++++++++++++++ > 2 files changed, 184 insertions(+) > create mode 100644 Documentation/userspace-api/iommufd.rst > > diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst > index 0b5eefed027e..54df5a278023 100644 > --- a/Documentation/userspace-api/index.rst > +++ b/Documentation/userspace-api/index.rst > @@ -25,6 +25,7 @@ place where this information is gathered. > ebpf/index > ioctl/index > iommu > + iommufd > media/index > sysfs-platform_profile > > diff --git a/Documentation/userspace-api/iommufd.rst b/Documentation/userspace-api/iommufd.rst > new file mode 100644 > index 000000000000..abffbb47dc02 > --- /dev/null > +++ b/Documentation/userspace-api/iommufd.rst > @@ -0,0 +1,183 @@ > +.. SPDX-License-Identifier: GPL-2.0 > +.. iommu: > + > +=================== > +IOMMU Userspace API > +=================== > + > +Direct device access from userspace has been a crtical feature in > +high performance computing and virtualization usages. Linux now > +includes multiple device-passthrough frameworks (e.g. VFIO and vDPA) > +to manage secure device access from the userspace. One critical > +task of those frameworks is to put the assigned device in a secure, > +IOMMU-protected context so the device is prevented from doing harm > +to the rest of the system. > + > +Currently those frameworks implement their own logic for managing > +I/O page tables to isolate user-initiated DMAs. This doesn't scale > +to support many new IOMMU features, such as PASID-granular DMA > +remapping, nested translation, I/O page fault, IOMMU dirty bit, etc. > + > +The /dev/iommu framework provides an unified interface for managing > +I/O page tables for passthrough devices. Existing passthrough > +frameworks are expected to use this interface instead of continuing > +their ad-hoc implementations. > + > +IOMMUFDs, IOASIDs, Devices and Groups > +------------------------------------- > + > +The core concepts in /dev/iommu are IOMMUFDs and IOASIDs. IOMMUFD (by > +opening /dev/iommu) is the container holding multiple I/O address > +spaces for a user, while IOASID is the fd-local software handle > +representing an I/O address space and associated with a single I/O > +page table. User manages those address spaces through fd operations, > +e.g. by using vfio type1v2 mapping semantics to manage respective > +I/O page tables. > + > +IOASID is comparable to the conatiner concept in VFIO. The latter > +is also associated to a single I/O address space. A main difference > +between them is that multiple IOASIDs in the same IOMMUFD can be > +nested together (not supported yet) to allow centralized accounting > +of locked pages, while multiple containers are disconnected thus > +duplicated accounting is incurred. Typically one IOMMUFD is > +sufficient for all intended IOMMU usages for a user. > + > +An I/O address space takes effect in the IOMMU only after it is > +attached by a device. One I/O address space can be attached by > +multiple devices. One device can be only attached to a single I/O > +address space at this point (on par with current vfio behavior). > + > +Device must be bound to an iommufd before the attach operation can > +be conducted. The binding operation builds the connection between > +the devicefd (opened via device-passthrough framework) and IOMMUFD. > +IOMMU-protected security context is esbliashed when the binding > +operation is completed. This can't be quite right. You can't establish a safe security context until all devices in the groun are bound, but you can only bind them one at a time. > The passthrough framework must block user > +access to the assigned device until bind() returns success. > + > +The entire /dev/iommu framework adopts a device-centric model w/o > +carrying any container/group legacy as current vfio does. However > +the group is the minimum granularity that must be used to ensure > +secure user access (refer to vfio.rst). This framework relies on > +the IOMMU core layer to map device-centric model into group-granular > +isolation. > + > +Managing I/O Address Spaces > +--------------------------- > + > +When creating an I/O address space (by allocating IOASID), the user > +must specify the type of underlying I/O page table. Currently only > +one type (kernel-managed) is supported. In the future other types > +will be introduced, e.g. to support user-managed I/O page table or > +a shared I/O page table which is managed by another kernel sub- > +system (mm, ept, etc.). Kernel-managed I/O page table is currently > +managed via vfio type1v2 equivalent mapping semantics. > + > +The user also needs to specify the format of the I/O page table > +when allocating an IOASID. This almost seems redundant with the previous paragraph. I think maybe it's making a distinction between "type" and "format", but I don't think it's very clear what the distinction is. > The format must be compatible to the > +attached devices (or more specifically to the IOMMU which serves > +the DMA from the attached devices). User can query the device IOMMU > +format via IOMMUFD once a device is successfully bound. Attaching a > +device to an IOASID with incompatible format is simply rejected. > + > +Currently no-snoop DMA is not supported yet. This implies that > +IOASID must be created in an enforce-snoop format and only devices > +which can be forced to snoop cache by IOMMU are allowed to be > +attached to IOASID. The user should check uAPI extension and get > +device info via IOMMUFD to handle such restriction. > + > +Usage Example > +------------- > + > +Assume user wants to access PCI device 0000:06:0d.0, which is > +exposed under the new /dev/vfio/devices directory by VFIO: > + > + /* Open device-centric interface and /dev/iommu interface */ > + device_fd = open("/dev/vfio/devices/0000:06:0d.0", O_RDWR); > + iommu_fd = open("/dev/iommu", O_RDWR); > + > + /* Bind device to IOMMUFD */ > + bind_data = { .iommu_fd = iommu_fd, .dev_cookie = cookie }; > + ioctl(device_fd, VFIO_DEVICE_BIND_IOMMUFD, &bind_data); > + > + /* Query per-device IOMMU capability/format */ > + info = { .dev_cookie = cookie, }; > + ioctl(iommu_fd, IOMMU_DEVICE_GET_INFO, &info); > + > + if (!(info.flags & IOMMU_DEVICE_INFO_ENFORCE_SNOOP)) { > + if (!ioctl(iommu_fd, IOMMU_CHECK_EXTENSION, > + EXT_DMA_NO_SNOOP)) > + /* No support of no-snoop DMA */ > + } > + > + if (!ioctl(iommu_fd, IOMMU_CHECK_EXTENSION, EXT_MAP_TYPE1V2)) > + /* No support of vfio type1v2 mapping semantics */ > + > + /* Decides IOASID alloc fields based on info */ > + alloc_data = { .type = IOMMU_IOASID_TYPE_KERNEL, > + .flags = IOMMU_IOASID_ENFORCE_SNOOP, > + .addr_width = info.addr_width, }; > + > + /* Allocate IOASID */ > + gpa_ioasid = ioctl(iommu_fd, IOMMU_IOASID_ALLOC, &alloc_data); > + > + /* Attach device to an IOASID */ > + at_data = { .iommu_fd = iommu_fd; .ioasid = gpa_ioasid}; > + ioctl(device_fd, VFIO_DEVICE_ATTACH_IOASID, &at_data); > + > + /* Setup GPA mapping [0 - 1GB] */ > + dma_map = { > + .ioasid = gpa_ioasid, > + .data { > + .flags = R/W /* permission */ > + .iova = 0, /* GPA */ > + .vaddr = 0x40000000, /* HVA */ > + .size = 1GB, > + }, > + }; > + ioctl(iommu_fd, IOMMU_MAP_DMA, &dma_map); > + > + /* DMA */ > + > + /* Unmap GPA mapping [0 - 1GB] */ > + dma_unmap = { > + .ioasid = gpa_ioasid, > + .data { > + .iova = 0, /* GPA */ > + .size = 1GB, > + }, > + }; > + ioctl(iommu_fd, IOMMU_UNMAP_DMA, &dma_unmap); > + > + /* Detach device from an IOASID */ > + dt_data = { .iommu_fd = iommu_fd; .ioasid = gpa_ioasid}; > + ioctl(device_fd, VFIO_DEVICE_DETACH_IOASID, &dt_data); > + > + /* Free IOASID */ > + ioctl(iommu_fd, IOMMU_IOASID_FREE, gpa_ioasid); > + > + close(device_fd); > + close(iommu_fd); > + > +API for device-passthrough frameworks > +------------------------------------- > + > +iommufd binding and IOASID attach/detach are initiated via the device- > +passthrough framework uAPI. > + > +When a binding operation is requested by the user, the passthrough > +framework should call iommufd_bind_device(). When the device fd is > +closed by the user, iommufd_unbind_device() should be called > +automatically:: > + > + struct iommufd_device * > + iommufd_bind_device(int fd, struct device *dev, > + u64 dev_cookie); > + void iommufd_unbind_device(struct iommufd_device *idev); > + > +IOASID attach/detach operations are per iommufd_device which is > +returned by iommufd_bind_device(): > + > + int iommufd_device_attach_ioasid(struct iommufd_device *idev, > + int ioasid); > + void iommufd_device_detach_ioasid(struct iommufd_device *idev, > + int ioasid); -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson