[Intel-gfx] [PATCH v9 25/25] docs: vfio: Add vfio device cdev description

Wed Apr 5 14:00:00 UTC 2023

Hi Eric,

> From: Eric Auger <eric.auger at redhat.com>
> Sent: Wednesday, April 5, 2023 9:46 PM
> 
> Hi Yi,
> 
> On 4/1/23 17:18, Yi Liu wrote:
> > This gives notes for userspace applications on device cdev usage.
> >
> > Reviewed-by: Kevin Tian <kevin.tian at intel.com>
> > Signed-off-by: Yi Liu <yi.l.liu at 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..4f21be7bda8a 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.
> > +For noiommu devices, the character device would be named with "noiommu-"
> > +prefix. e.g. /dev/vfio/devices/noiommu-vfioX.
> > +
> > +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.
> > +
> > +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
> you mentionned in the pci hot reset series that the BDF couldn't be used
> if cdev is being used. According to the above, it could, no?

It should be the device passing case, otherwise, BDF can be used. But
from kernel p.o.v., it has no idea how user gets the device fd, so it needs
to assume user may not have BDF knowledge. 

> > +	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; // negative value means vfio-noiommu mode
> > +	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" */
> dev_id is not mentioned anywhere whereas it is used in the reset get
> info. Worth to be mentioned?

a good point. but it is just for PCI devices, not sure if it is good to
mention it here. or the kdoc in the HOT_RESET_INFO ioctl is already
enough. Alex, how about your taste?.

Regards,
Yi Liu

> 
> Otherwise looks good to me
> 
> Eric
> > +
> >  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).