Re: [PATCH v12 24/24] docs: vfio: Add vfio device cdev description

Alex Williamson
2023-06-12T23:07:18Z

On Fri, 2 Jun 2023 05:16:53 -0700
Yi Liu <yi.l.liu@intel.com> wrote:

> This gives notes for userspace applications on device cdev usage.
>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> ---
> Documentation/driver-api/vfio.rst | 132 ++++++++++++++++++++++++++++++
> 1 file changed, 132 insertions(+)
>
> diff --git a/Documentation/driver-api/vfio.rst b/Documentation/driver-api/vfio.rst
> index 363e12c90b87..f00c9b86bda0 100644
> --- a/Documentation/driver-api/vfio.rst
> +++ b/Documentation/driver-api/vfio.rst
> @@ -239,6 +239,130 @@ group and can access them as follows::
> /* Gratuitous device reset and go... */
> ioctl(device, VFIO_DEVICE_RESET);
>
> +IOMMUFD and vfio_iommu_type1
> +----------------------------
> +
> +IOMMUFD is the new user API to manage I/O page tables from userspace.
> +It intends to be the portal of delivering advanced userspace DMA
> +features (nested translation [5]_, PASID [6]_, etc.) while also providing
> +a backwards compatibility interface for existing VFIO_TYPE1v2_IOMMU use
> +cases. Eventually the vfio_iommu_type1 driver, as well as the legacy
> +vfio container and group model is intended to be deprecated.
> +
> +The IOMMUFD backwards compatibility interface can be enabled two ways.
> +In the first method, the kernel can be configured with
> +CONFIG_IOMMUFD_VFIO_CONTAINER, in which case the IOMMUFD subsystem
> +transparently provides the entire infrastructure for the VFIO
> +container and IOMMU backend interfaces. The compatibility mode can
> +also be accessed if the VFIO container interface, ie. /dev/vfio/vfio is
> +simply symlink'd to /dev/iommu. Note that at the time of writing, the
> +compatibility mode is not entirely feature complete relative to
> +VFIO_TYPE1v2_IOMMU (ex. DMA mapping MMIO) and does not attempt to
> +provide compatibility to the VFIO_SPAPR_TCE_IOMMU interface. Therefore
> +it is not generally advisable at this time to switch from native VFIO
> +implementations to the IOMMUFD compatibility interfaces.
> +
> +Long term, VFIO users should migrate to device access through the cdev
> +interface described below, and native access through the IOMMUFD
> +provided interfaces.
> +
> +VFIO Device cdev
> +----------------
> +
> +Traditionally user acquires a device fd via VFIO_GROUP_GET_DEVICE_FD
> +in a VFIO group.
> +
> +With CONFIG_VFIO_DEVICE_CDEV=y the user can now acquire a device fd
> +by directly opening a character device /dev/vfio/devices/vfioX where
> +"X" is the number allocated uniquely by VFIO for registered devices.
> +cdev interface does not support noiommu, so user should use the legacy
> +group interface if noiommu is needed.
> +
> +The cdev only works with IOMMUFD. Both VFIO drivers and applications
> +must adapt to the new cdev security model which requires using
> +VFIO_DEVICE_BIND_IOMMUFD to claim DMA ownership before starting to
> +actually use the device. Once BIND succeeds then a VFIO device can
> +be fully accessed by the user.
> +
> +VFIO device cdev doesn't rely on VFIO group/container/iommu drivers.
> +Hence those modules can be fully compiled out in an environment
> +where no legacy VFIO application exists.
> +
> +So far SPAPR does not support IOMMUFD yet. So it cannot support device
> +cdev neither.

s/neither/either/

Unless I missed it, we've not described that vfio device cdev access is
still bound by IOMMU group semantics, ie. there can be one DMA owner
for the group. That's a pretty common failure point for multi-function
consumer device use cases, so the why, where, and how it fails should
be well covered.

In general there's been a lot of cross collaboration to get the series
this far. I see an abundance of Tested-by, but unfortunately not a lot
of Reviewed-by beyond about the first 1/3rd of the series. Thanks,

Alex

> +
> +Device cdev Example
> +-------------------
> +
> +Assume user wants to access PCI device 0000:6a:01.0::
> +
> + $ ls /sys/bus/pci/devices/0000:6a:01.0/vfio-dev/
> + vfio0
> +
> +This device is therefore represented as vfio0. The user can verify
> +its existence::
> +
> + $ ls -l /dev/vfio/devices/vfio0
> + crw------- 1 root root 511, 0 Feb 16 01:22 /dev/vfio/devices/vfio0
> + $ cat /sys/bus/pci/devices/0000:6a:01.0/vfio-dev/vfio0/dev
> + 511:0
> + $ ls -l /dev/char/511\:0
> + lrwxrwxrwx 1 root root 21 Feb 16 01:22 /dev/char/511:0 -> ../vfio/devices/vfio0
> +
> +Then provide the user with access to the device if unprivileged
> +operation is desired::
> +
> + $ chown user:user /dev/vfio/devices/vfio0
> +
> +Finally the user could get cdev fd by::
> +
> + cdev_fd = open("/dev/vfio/devices/vfio0", O_RDWR);
> +
> +An opened cdev_fd doesn't give the user any permission of accessing
> +the device except binding the cdev_fd to an iommufd. After that point
> +then the device is fully accessible including attaching it to an
> +IOMMUFD IOAS/HWPT to enable userspace DMA::
> +
> + struct vfio_device_bind_iommufd bind = {
> + .argsz = sizeof(bind),
> + .flags = 0,
> + };
> + struct iommu_ioas_alloc alloc_data = {
> + .size = sizeof(alloc_data),
> + .flags = 0,
> + };
> + struct vfio_device_attach_iommufd_pt attach_data = {
> + .argsz = sizeof(attach_data),
> + .flags = 0,
> + };
> + struct iommu_ioas_map map = {
> + .size = sizeof(map),
> + .flags = IOMMU_IOAS_MAP_READABLE |
> + IOMMU_IOAS_MAP_WRITEABLE |
> + IOMMU_IOAS_MAP_FIXED_IOVA,
> + .__reserved = 0,
> + };
> +
> + iommufd = open("/dev/iommu", O_RDWR);
> +
> + bind.iommufd = iommufd;
> + ioctl(cdev_fd, VFIO_DEVICE_BIND_IOMMUFD, &bind);
> +
> + ioctl(iommufd, IOMMU_IOAS_ALLOC, &alloc_data);
> + attach_data.pt_id = alloc_data.out_ioas_id;
> + ioctl(cdev_fd, VFIO_DEVICE_ATTACH_IOMMUFD_PT, &attach_data);
> +
> + /* Allocate some space and setup a DMA mapping */
> + map.user_va = (int64_t)mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
> + MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
> + map.iova = 0; /* 1MB starting at 0x0 from device view */
> + map.length = 1024 * 1024;
> + map.ioas_id = alloc_data.out_ioas_id;;
> +
> + ioctl(iommufd, IOMMU_IOAS_MAP, &map);
> +
> + /* Other device operations as stated in "VFIO Usage Example" */
> +
> VFIO User API
> -------------------------------------------------------------------------------
>
> @@ -566,3 +690,11 @@ This implementation has some specifics:
> \-0d.1
>
> 00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
> +
> +.. [5] Nested translation is an IOMMU feature which supports two stage
> + address translations. This improves the address translation efficiency
> + in IOMMU virtualization.
> +
> +.. [6] PASID stands for Process Address Space ID, introduced by PCI
> + Express. It is a prerequisite for Shared Virtual Addressing (SVA)
> + and Scalable I/O Virtualization (Scalable IOV).