RFC: vfio interface for platform devices (v2)

Alex Williamson
2013-07-03T22:53:09Z

On Wed, 2013-07-03 at 21:40 +0000, Yoder Stuart-B08248
wrote:
> Version 2
>   -VFIO_GROUP_GET_DEVICE_FD-- specified that the path is a sysfs path
>   -VFIO_DEVICE_GET_INFO-- defined 2 flags instead of 1
>   -deleted VFIO_DEVICE_GET_DEVTREE_INFO ioctl
>   -VFIO_DEVICE_GET_REGION_INFO-- updated as per AlexW's suggestion,
>    defined 5 new flags and associated structs
>   -VFIO_DEVICE_GET_IRQ_INFO-- updated as per AlexW's suggestion,
>    defined 1 new flag and associated struct
>   -removed redundant example
>
>
------------------------------------------------------------------------------
> VFIO for Platform Devices
>
> The existing kernel interface for vfio-pci is pretty close to what is
needed
> for platform devices:
>    -mechanism to create a container
>    -add groups/devices to a container
>    -set the IOMMU model
>    -map DMA regions
>    -get an fd for a specific device, which allows user space to determine
>     info about device regions (e.g. registers) and interrupt info
>    -support for mmapping device regions
>    -mechanism to set how interrupts are signaled
>
> Many platform device are simple and consist of a single register
> region and a single interrupt.  For these types of devices the
> existing vfio interfaces should be sufficient.
>
> However, platform devices can get complicated-- logically represented
> as a device tree hierarchy of nodes.  For devices with multiple regions
> and interrupts, new mechanisms are needed in vfio to correlate the
> regions/interrupts with the device tree structure that drivers use
> to determine the meaning of device resources.
>
> In some cases there are relationships between device, and devices
> reference other devices using phandle links.  The kernel won't expose
> relationships between devices, but just exposes mappable register
> regions and interrupts.
>
> The changes needed for vfio are around some of the device tree
> related info that needs to be available with the device fd.
>
> 1.  VFIO_GROUP_GET_DEVICE_FD
>
>   User space knows by out-of-band means which device it is accessing
>   and will call VFIO_GROUP_GET_DEVICE_FD passing a specific sysfs path
>   to get the device information:
>
>   fd = ioctl(group, VFIO_GROUP_GET_DEVICE_FD,
>              "/sys/bus/platform/devices/ffe210000.usb"));

FWIW, I'm in favor of whichever way works out cleaner in the code for
pre-pending "/sys/bus" or not.  It sort of seems like it's
unnecessary.
It's also a little inconsistent that the returned path doesn't
pre-pend /sys in the examples below.

> 2.  VFIO_DEVICE_GET_INFO
>
>    The number of regions corresponds to the regions defined
>    in "reg" and "ranges" in the device tree.
>
>    Two new flags are added to struct vfio_device_info:
>
>    #define VFIO_DEVICE_FLAGS_PLATFORM (1 << ?) /* A platform bus
device */
>    #define VFIO_DEVICE_FLAGS_DEVTREE  (1 << ?) /* device tree info
available */
>
>    It is possible that there could be platform bus devices
>    that are not in the device tree, so we use 2 flags to
>    allow for that.
>
>    If just VFIO_DEVICE_FLAGS_PLATFORM is set, it means
>    that there are regions and IRQs but no device tree info
>    available.
>
>    If just VFIO_DEVICE_FLAGS_DEVTREE is set, it means
>    there is device tree info available.

But it would be invalid to only have DEVTREE w/o PLATFORM for now,
right?

> 3. VFIO_DEVICE_GET_REGION_INFO
>
>    For platform devices with multiple regions, information
>    is needed to correlate the regions with the device
>    tree structure that drivers use to determine the meaning
>    of device resources.
>
>    The VFIO_DEVICE_GET_REGION_INFO is extended to provide
>    device tree information.
>
>    The following information is needed:
>       -the device tree path to the node corresponding to the
>        region
>       -whether it corresponds to a "reg" or "ranges"
property
>       -there could be multiple sub-regions per "reg" or
"ranges" and
>        the sub-index within the reg/ranges is needed
>
>    There are 5 new flags added to vfio_region_info :
>
>    struct vfio_region_info {
>         __u32   argsz;
>         __u32   flags;
>    #define VFIO_REGION_INFO_FLAG_CACHEABLE (1 << ?)
>    #define VFIO_DEVTREE_REGION_INFO_FLAG_REG (1 << ?)
>    #define VFIO_DEVTREE_REGION_INFO_FLAG_RANGE (1 << ?)
>    #define VFIO_DEVTREE_REGION_INFO_FLAG_INDEX (1 << ?)
>    #define VFIO_DEVTREE_REGION_INFO_FLAG_PATH (1 << ?)
>         __u32   index;          /* Region index */
>         __u32   resv;           /* Reserved for alignment */
>         __u64   size;           /* Region size (bytes) */
>         __u64   offset;         /* Region offset from start of device fd */
>    };
>
>    VFIO_REGION_INFO_FLAG_CACHEABLE
>        -if set indicates that the region must be mapped as cacheable
>
>    VFIO_DEVTREE_REGION_INFO_FLAG_REG
>        -if set indicates that the region corresponds to a "reg"
property
>         in the device tree representation of the device
>
>    VFIO_DEVTREE_REGION_INFO_FLAG_RANGE
>        -if set indicates that the region corresponds to a
"ranges" property
>         in the device tree representation of the device
>
>    VFIO_DEVTREE_REGION_INFO_FLAG_INDEX
>        -if set indicates that there is a dword aligned struct
>         struct vfio_devtree_region_info_index appended to the
>         end of vfio_region_info:
>
>         struct vfio_devtree_region_info_index
>         {
> 	      u32 index;
>         }
>
>         A reg or ranges property may have multiple regsion.  The index
>         specifies the index within the "reg" or
"ranges"
>         that this region corresponds to.
>
>    VFIO_DEVTREE_REGION_INFO_FLAG_PATH
>        -if set indicates that there is a dword aligned struct
>         struct vfio_devtree_info_path appended to the
>         end of vfio_region_info:
>
>         struct vfio_devtree_info_path
>         {
>             u32 len;
>             u8 path[];
>         }
>
>         The path is the full path to the corresponding device
>         tree node.  The len field specifies the length of the
>         path string.
>
>    If multiple flags are set that indicate that there is
>    an appended struct, the order of the flags indicates
>    the order of the structs.
>
>    argsz is set by the kernel specifying the total size of
>    struct vfio_region_info and all appended structs.
>
>    Suggested usage:
>       -call VFIO_DEVICE_GET_REGION_INFO with argsz >
sizeof(struct vfio_region_info)
>       -realloc the buffer
>       -call VFIO_DEVICE_GET_REGION_INFO again, and the appended
>        structs will be returned
>
> 4.  VFIO_DEVICE_GET_IRQ_INFO
>
>    For platform devices with multiple interrupts that
>    correspond to different subnodes in the device tree,
>    information is needed to correlate the interrupts
>    to the the device tree structure.
>
>    The VFIO_DEVICE_GET_REGION_INFO is extended to provide
>    device tree information.
>
>    1 new flag is added to vfio_irq_info :
>
>    struct vfio_irq_info {
>         __u32   argsz;
>         __u32   flags;
>    #define VFIO_DEVTREE_IRQ_INFO_FLAG_PATH (1 << ?)
>         __u32   index;    /* IRQ index */
>         __u32   count;    /* Number of IRQs within this index */
>     };
>
>    VFIO_DEVTREE_IRQ_INFO_FLAG_PATH
>        -if set indicates that there is a dword aligned struct
>         struct vfio_devtree_info_path appended to the
>         end of vfio_irq_info :
>
>         struct vfio_devtree_info_path
>         {
>             u32 len;
>             u8 path[];
>         }
>
>         The path is the full path to the corresponding device
>         tree node.  The len field specifies the length of the
>         path string.
>
>    argsz is set by the kernel specifying the total size of
>    struct vfio_region_info and all appended structs.
>
> 5.  EXAMPLE 1
>
>     Example, Freescale SATA controller:
>
>      sata at 220000 {
>          compatible = "fsl,p2041-sata",
"fsl,pq-sata-v2";
>          reg = <0x220000 0x1000>;
>          interrupts = <0x44 0x2 0x0 0x0>;
>      };
>
>     request to get device FD would look like:
>       fd = ioctl(group, VFIO_GROUP_GET_DEVICE_FD,
"/sys/bus/platform/devices/ffe220000.sata");
>
>     The VFIO_DEVICE_GET_INFO ioctl would return:
>       -1 region
>       -1 interrupts
>
>     The VFIO_DEVICE_GET_REGION_INFO ioctl would return:
>       -for index 0:
>            offset=0, size=0x10000 -- allows mmap of physical 0xffe220000
>            flags = VFIO_DEVTREE_REGION_INFO_FLAG_REG |
>                    VFIO_DEVTREE_REGION_INFO_FLAG_PATH
>            vfio_devtree_info_path
>               len = 26
>               path = "/soc at ffe000000/sata at 220000"
>
>     The VFIO_DEVICE_GET_IRQ_INFO ioctl would return:
>       -for index 0:
>           flags = VFIO_IRQ_INFO_EVENTFD |
>                   VFIO_IRQ_INFO_MASKABLE |
>                   VFIO_DEVTREE_IRQ_INFO_FLAG_PATH
>           vfio_devtree_info_path
>               len = 26
>               path = "/soc at ffe000000/sata at 220000"
>
> 6.  EXAMPLE 2
>
>     Example, Freescale DMA engine (modified to illustrate):
>
>     dma at 101300 {
>        cell-index = <0x1>;
>        ranges = <0x0 0x101100 0x200>;
>        reg = <0x101300 0x4>;
>        compatible = "fsl,eloplus-dma";
>        #size-cells = <0x1>;
>        #address-cells = <0x1>;
>        fsl,liodn = <0xc6>;
>
>        dma-channel at 180 {
>           interrupts = <0x23 0x2 0x0 0x0>;
>           cell-index = <0x3>;
>           reg = <0x180 0x80>;
>           compatible = "fsl,eloplus-dma-channel";
>        };
>
>        dma-channel at 100 {
>           interrupts = <0x22 0x2 0x0 0x0>;
>           cell-index = <0x2>;
>           reg = <0x100 0x80>;
>           compatible = "fsl,eloplus-dma-channel";
>        };
>
>     };
>
>     request to get device FD would look like:
>       fd = ioctl(group, VFIO_GROUP_GET_DEVICE_FD,
"/sys/bus/platform/devices/ffe101300.dma");
>
>     The VFIO_DEVICE_GET_INFO ioctl would return:
>       -2 regions
>       -2 interrupts
>
>     The VFIO_DEVICE_GET_REGION_INFO ioctl would return:
>       -for index 0:
>            offset=0x100, size=0x200 -- allows mmap of physical 0xffe101100
>            flags = VFIO_DEVTREE_REGION_INFO_FLAG_RANGES |
>                    VFIO_DEVTREE_REGION_INFO_FLAG_PATH
>            vfio_devtree_info_path
>               len = 25
>               path = "/soc at ffe000000/dma at 101300"
>
>       -for index 1:
>            offset=0x300, size=0x4 -- allows mmap of physical 0xffe101300
>            flags = VFIO_DEVTREE_REGION_INFO_FLAG_REG |
>                    VFIO_DEVTREE_REGION_INFO_FLAG_PATH
>            vfio_devtree_info_path
>               len = 25
>               path = "/soc at ffe000000/dma at 101300"
>
>     The VFIO_DEVICE_GET_IRQ_INFO ioctl would return:
>       -for index 0:
>           flags = VFIO_IRQ_INFO_EVENTFD |
>                   VFIO_IRQ_INFO_MASKABLE |
>                   VFIO_DEVTREE_IRQ_INFO_FLAG_PATH
>           vfio_devtree_info_path
>               len = 41
>               path = "/soc at ffe000000/dma at 101300/dma-channel at
180"
>
>       -for index 0:
>           flags = VFIO_IRQ_INFO_EVENTFD |
>                   VFIO_IRQ_INFO_MASKABLE |
>                   VFIO_DEVTREE_IRQ_INFO_FLAG_PATH
>           vfio_devtree_info_path
>               len = 41
>               path = "/soc at ffe000000/dma at 101300/dma-channel at
100"


Seems like it should work.  My only API concern with this model of
appending structs is that a user needs to know the size of each struct
even if they don't otherwise care about it in order to step over it.  In
some cases, like the path, the size is variable and the user needs to
look into it.  The structs must also be strictly ordered based on the
order of the flags or all hope is lost.  If we assign flags sequentially
there should be no case where the user needs to step over something that
they doesn't know the size of.  Even so, we may still be ahead to define
the first word of each struct as the length (I'm guessing a byte might
be too limiting).  It would sure make walking it easier.  Thanks,

Alex