Re: Plan for /dev/ioasid RFC v2

Lu Baolu
2021-06-24T13:42:50Z

On 2021/6/24 12:03, David Gibson wrote:
> On Fri, Jun 18, 2021 at 01:21:47PM +0800, Lu Baolu wrote:
>> Hi David,
>>
>> On 6/17/21 1:22 PM, David Gibson wrote:
>>>> The iommu_group can guarantee the isolation among different physical
>>>> devices (represented by RIDs). But when it comes to sub-devices (ex. mdev or
>>>> vDPA devices represented by RID + SSID), we have to rely on the
>>>> device driver for isolation. The devices which are able to generate sub-
>>>> devices should either use their own on-device mechanisms or use the
>>>> platform features like Intel Scalable IOV to isolate the sub-devices.
>>> This seems like a misunderstanding of groups. Groups are not tied to
>>> any PCI meaning. Groups are the smallest unit of isolation, no matter
>>> what is providing that isolation.
>>>
>>> If mdevs are isolated from each other by clever software, even though
>>> they're on the same PCI device they are in different groups from each
>>> other*by definition*. They are also in a different group from their
>>> parent device (however the mdevs only exist when mdev driver is
>>> active, which implies that the parent device's group is owned by the
>>> kernel).
>>
>> You are right. This is also my understanding of an "isolation group".
>>
>> But, as I understand it, iommu_group is only the isolation group visible
>> to IOMMU. When we talk about sub-devices (sw-mdev or mdev w/ pasid),
>> only the device and device driver knows the details of isolation, hence
>> iommu_group could not be extended to cover them. The device drivers
>> should define their own isolation groups.
> So, "iommu group" isn't a perfect name. It came about because
> originally the main mechanism for isolation was the IOMMU, so it was
> typically the IOMMU's capabilities that determined if devices were
> isolated. However it was always known that there could be other
> reasons for failure of isolation. To simplify the model we decided
> that we'd put things into the same group if they were non-isolated for
> any reason.

Yes.

>
> The kernel has no notion of "isolation group" as distinct from "iommu
> group". What are called iommu groups in the kernel now*are*
> "isolation groups" and that was always the intention - it's just not a
> great name.

Fair enough.

>
>> Otherwise, the device driver has to fake an iommu_group and add hacky
>> code to link the related IOMMU elements (iommu device, domain, group
>> etc.) together. Actually this is part of the problem that this proposal
>> tries to solve.
> Yeah, that's not ideal.
>
>>>> Under above conditions, different sub-device from a same RID device
>>>> could be able to use different IOASID. This seems to means that we can't
>>>> support mixed mode where, for example, two RIDs share an iommu_group and
>>>> one (or both) of them have sub-devices.
>>> That doesn't necessarily follow. mdevs which can be successfully
>>> isolated by their mdev driver are in a different group from their
>>> parent device, and therefore need not be affected by whether the
>>> parent device shares a group with some other physical device. They
>>> *might* be, but that's up to the mdev driver to determine based on
>>> what it can safely isolate.
>>>
>> If we understand it as multiple levels of isolation, can we classify the
>> devices into the following categories?
>>
>> 1) Legacy devices
>> - devices without device-level isolation
>> - multiple devices could sit in a single iommu_group
>> - only a single I/O address space could be bound to IOMMU
> I'm not really clear on what that last statement means.

I mean a single iommu_domain should be used by all devices sharing a
single iommu_group.

>
>> 2) Modern devices
>> - devices capable of device-level isolation
> This will*typically* be true of modern devices, but I don't think we
> can really make it a hard API distinction. Legacy or buggy bridges
> can force modern devices into the same group as each other. Modern
> devices are not immune from bugs which would force lack of isolation
> (e.g. forgotten debug registers on function 0 which affect other
> functions).
>

Yes.

I am thinking whether it's feasible to change "bind/attach a device to
an IOASID" to "bind/attach an isolated unit to an IOASID". An isolated
unit could be

1) an iommu_ group including single or multiple devices;
2) a physical device which have a 1-device iommu group + device ID
(PASID/subStreamID) which represents an isolated subdevice inside the
physical one.
3) anything that we might have in the future.

A handler which represents the connection between device and iommu is
returned on any successful binding. This handler could be used to
GET_INFO and attach/detach after binding.

Best regards,
baolu