Re: [RFC] /dev/ioasid uAPI proposal

Jason Gunthorpe
2021-05-31T18:09:22Z

On Mon, May 31, 2021 at 07:31:57PM +0800, Liu Yi L wrote:
> > > /*
> > > * Get information about an I/O address space
> > > *
> > > * Supported capabilities:
> > > * - VFIO type1 map/unmap;
> > > * - pgtable/pasid_table binding
> > > * - hardware nesting vs. software nesting;
> > > * - ...
> > > *
> > > * Related attributes:
> > > * - supported page sizes, reserved IOVA ranges (DMA mapping);
> > > * - vendor pgtable formats (pgtable binding);
> > > * - number of child IOASIDs (nesting);
> > > * - ...
> > > *
> > > * Above information is available only after one or more devices are
> > > * attached to the specified IOASID. Otherwise the IOASID is just a
> > > * number w/o any capability or attribute.
> >
> > This feels wrong to learn most of these attributes of the IOASID after
> > attaching to a device.
>
> but an IOASID is just a software handle before attached to a specific
> device. e.g. before attaching to a device, we have no idea about the
> supported page size in underlying iommu, coherent etc.

The idea is you attach the device to the /dev/ioasid FD and this
action is what crystalizes the iommu driver that is being used:

device_fd = open("/dev/vfio/devices/dev1", mode);
ioasid_fd = open("/dev/ioasid", mode);
ioctl(device_fd, VFIO_BIND_IOASID_FD, ioasid_fd);

After this sequence we should have most of the information about the
IOMMU.

One /dev/ioasid FD has one iommu driver. Design what an "iommu driver"
means so that the system should only have one. Eg the coherent/not
coherent distinction should not be a different "iommu driver".

Device attach to the _IOASID_ is a different thing, and I think it
puts the whole sequence out of order because we loose the option to
customize the IOASID before it has to be realized into HW format.

> > The user should have some idea how it intends to use the IOASID when
> > it creates it and the rest of the system should match the intention.
> >
> > For instance if the user is creating a IOASID to cover the guest GPA
> > with the intention of making children it should indicate this during
> > alloc.
> >
> > If the user is intending to point a child IOASID to a guest page table
> > in a certain descriptor format then it should indicate it during
> > alloc.
>
> Actually, we have only two kinds of IOASIDs so far.

Maybe at a very very high level, but it looks like there is alot of
IOMMU specific configuration that goes into an IOASD.

> > device bind should fail if the device somehow isn't compatible with
> > the scheme the user is tring to use.
>
> yeah, I guess you mean to fail the device attach when the IOASID is a
> nesting IOASID but the device is behind an iommu without nesting support.
> right?

Right..

> >
> > > /*
> > > * Map/unmap process virtual addresses to I/O virtual addresses.
> > > *
> > > * Provide VFIO type1 equivalent semantics. Start with the same
> > > * restriction e.g. the unmap size should match those used in the
> > > * original mapping call.
> > > *
> > > * If IOASID_REGISTER_MEMORY has been called, the mapped vaddr
> > > * must be already in the preregistered list.
> > > *
> > > * Input parameters:
> > > * - u32 ioasid;
> > > * - refer to vfio_iommu_type1_dma_{un}map
> > > *
> > > * Return: 0 on success, -errno on failure.
> > > */
> > > #define IOASID_MAP_DMA _IO(IOASID_TYPE, IOASID_BASE + 6)
> > > #define IOASID_UNMAP_DMA _IO(IOASID_TYPE, IOASID_BASE + 7)
> >
> > What about nested IOASIDs?
>
> at first glance, it looks like we should prevent the MAP/UNMAP usage on
> nested IOASIDs. At least hardware nested translation only allows MAP/UNMAP
> on the parent IOASIDs and page table bind on nested IOASIDs. But considering
> about software nesting, it seems still useful to allow MAP/UNMAP usage
> on nested IOASIDs. This is how I understand it, how about your opinion
> on it? do you think it's better to allow MAP/UNMAP usage only on parent
> IOASIDs as a start?

If the only form of nested IOASID is the "read the page table from
my process memory" then MAP/UNMAP won't make sense on that..

MAP/UNMAP is only useful if the page table is stored in kernel memory.

> > > #define IOASID_CREATE_NESTING _IO(IOASID_TYPE, IOASID_BASE + 8)
> >
> > Do you think another ioctl is best? Should this just be another
> > parameter to alloc?
>
> either is fine. This ioctl is following one of your previous comment.

Sometimes I say things in a way that is ment to be easier to
understand conecpts not necessarily good API design :)

> > > #define IOASID_BIND_PGTABLE _IO(IOASID_TYPE, IOASID_BASE + 9)
> > > #define IOASID_UNBIND_PGTABLE _IO(IOASID_TYPE, IOASID_BASE + 10)
> >
> > Also feels backwards, why wouldn't we specify this, and the required
> > page table format, during alloc time?
>
> here the model is user-space gets the page table format from kernel and
> decide if it can proceed. So what you are suggesting is user-space should
> tell kernel the page table format it has in ALLOC and kenrel should fail
> the ALLOC if the user-space page table format is not compatible with underlying
> iommu?

Yes, the action should be
Alloc an IOASID that points at a page table in this user memory,
that is stored in this specific format.

The supported formats should be discoverable after VFIO_BIND_IOASID_FD

> > > /*
> > > * Page fault report and response
> > > *
> > > * This is TBD. Can be added after other parts are cleared up. Likely it
> > > * will be a ring buffer shared between user/kernel, an eventfd to notify
> > > * the user and an ioctl to complete the fault.
> > > *
> > > * The fault data is per I/O address space, i.e.: IOASID + faulting_addr
> > > */
> >
> > Any reason not to just use read()?
>
> a ring buffer may be mmap to user-space, thus reading fault data from kernel
> would be faster. This is also how Eric's fault reporting is doing today.

Okay, if it is performance sensitive.. mmap rings are just tricky beasts

> > > * Bind a vfio_device to the specified IOASID fd
> > > *
> > > * Multiple vfio devices can be bound to a single ioasid_fd, but a single
> > > * vfio device should not be bound to multiple ioasid_fd's.
> > > *
> > > * Input parameters:
> > > * - ioasid_fd;
> > > *
> > > * Return: 0 on success, -errno on failure.
> > > */
> > > #define VFIO_BIND_IOASID_FD _IO(VFIO_TYPE, VFIO_BASE + 22)
> > > #define VFIO_UNBIND_IOASID_FD _IO(VFIO_TYPE, VFIO_BASE + 23)
> >
> > This is where it would make sense to have an output "device id" that
> > allows /dev/ioasid to refer to this "device" by number in events and
> > other related things.
>
> perhaps this is the device info Jean Philippe wants in page fault reporting
> path?

Yes, it is

Jason