Re: Plan for /dev/ioasid RFC v2

[Lu Baolu <baolu.lu@linux.intel.com>]

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.

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.

> 
>> 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

2) Modern devices
    - devices capable of device-level isolation
    - able to have subdevices
    - self-isolated, hence not share iommu_group with others
    - multiple I/O address spaces could be bound to IOMMU

For 1), all devices in an iommu_group should be bound to a single
IOASID; The isolation is guaranteed by an iommu_group.

For 2) a single device could be bound to multiple IOASIDs with each sub-
device corresponding to an IOASID. The isolation of each subdevice is
guaranteed by the device driver.

Best regards,
baolu

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