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SubjectRE: Plan for /dev/ioasid RFC v2
Date
> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Friday, June 25, 2021 10:36 PM
>
> On Fri, Jun 25, 2021 at 10:27:18AM +0000, Tian, Kevin wrote:
>
> > - When receiving the binding call for the 1st device in a group, iommu_fd
> > calls iommu_group_set_block_dma(group, dev->driver) which does
> > several things:
>
> The whole problem here is trying to match this new world where we want
> devices to be in charge of their own IOMMU configuration and the old
> world where groups are in charge.
>
> Inserting the group fd and then calling a device-centric
> VFIO_GROUP_GET_DEVICE_FD_NEW doesn't solve this conflict, and isn't
> necessary.

No, this was not what I meant. There is no group fd required when
calling this device-centric interface. I was actually talking about:

iommu_group_set_block_dma(dev->group, dev->driver)

just because current iommu layer API is group-centric. Whether this
should be improved could be next-level thing. Sorry for not making
it clear in the first place.

> We can always get the group back from the device at any
> point in the sequence do to a group wide operation.

yes.

>
> What I saw as the appeal of the sort of idea was to just completely
> leave all the difficult multi-device-group scenarios behind on the old
> group centric API and then we don't have to deal with them at all, or
> least not right away.

yes, this is the staged approach that we discussed earlier. and
the reason why I refined this proposal about multi-devices group
here is because you want to see some confidence along this
direction. Thus I expanded your idea and hope to achieve consensus
with Alex/Joerg who obviously have not been convinced yet.

>
> I'd see some progression where iommu_fd only works with 1:1 groups at
> the start. Other scenarios continue with the old API.

One uAPI open after completing this new sketch. v1 proposed to
conduct binding (VFIO_BIND_IOMMU_FD) after device_fd is acquired.
With this sketch we need a new VFIO_GROUP_GET_DEVICE_FD_NEW
to complete both in one step. I want to get Alex's confirmation whether
it sounds good to him, since it's better to unify the uAPI between 1:1
group and 1:N group even if we don't support 1:N in the start.

>
> Then maybe groups where all devices use the same IOASID.
>
> Then 1:N groups if the source device is reliably identifiable, this
> requires iommu subystem work to attach domains to sub-group objects -
> not sure it is worthwhile.
>
> But at least we can talk about each step with well thought out patches
>
> The only thing that needs to be done to get the 1:1 step is to broadly
> define how the other two cases will work so we don't get into trouble
> and set some way to exclude the problematic cases from even getting to
> iommu_fd in the first place.
>
> For instance if we go ahead and create /dev/vfio/device nodes we could
> do this only if the group was 1:1, otherwise the group cdev has to be
> used, along with its API.

I feel for VFIO possibly we don't need significant change to its uAPI
sequence, since it anyway needs to support existing semantics for
backward compatibility. With this sketch we can keep vfio container/
group by introducing an external iommu type which implies a different
GET_DEVICE_FD semantics. /dev/iommu can report a fd-wide capability
for whether 1:N group is supported to vfio user.

For new subsystems they can directly create device nodes and rely on
iommu fd to manage group isolation, without introducing any group
semantics in its uAPI.

>
> > a) Check group viability. A group is viable only when all devices in
> > the group are in one of below states:
> >
> > * driver-less
> > * bound to a driver which is same as dev->driver (vfio in this case)
> > * bound to an otherwise allowed driver (same list as in vfio)
>
> This really shouldn't use hardwired driver checks. Attached drivers
> should generically indicate to the iommu layer that they are safe for
> iommu_fd usage by calling some function around probe()

good idea.

>
> Thus a group must contain only iommu_fd safe drivers, or drivers-less
> devices before any of it can be used. It is the more general
> refactoring of what VFIO is doing.
>
> > c) The iommu layer also verifies group viability on BUS_NOTIFY_
> > BOUND_DRIVER event. BUG_ON if viability is broken while
> block_dma
> > is set.
>
> And with this concept of iommu_fd safety being first-class maybe we
> can somehow eliminate this gross BUG_ON (and the 100's of lines of
> code that are used to create it) by denying probe to non-iommu-safe
> drivers, somehow.

yes.

>
> > - Binding other devices in the group to iommu_fd just succeeds since
> > the group is already in block_dma.
>
> I think the rest of this more or less describes the device centric
> logic for multi-device groups we've already talked about. I don't
> think it benifits from having the group fd
>

sure. All of this new sketch doesn't have group fd in any iommu fd
API. Just try to elaborate a full sketch to sync the base.

Alex/Joerg, look forward to your thoughts now. 😊

Thanks
Kevin
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