Re: [RFC] /dev/ioasid uAPI proposal

Lu Baolu
2021-06-01T12:30:48Z

On 2021/6/1 15:15, Shenming Lu wrote:
> On 2021/6/1 13:10, Lu Baolu wrote:
>> Hi Shenming,
>>
>> On 6/1/21 12:31 PM, Shenming Lu wrote:
>>> On 2021/5/27 15:58, Tian, Kevin wrote:
>>>> /dev/ioasid provides an unified interface for managing I/O page tables for
>>>> devices assigned to userspace. Device passthrough frameworks (VFIO, vDPA,
>>>> etc.) are expected to use this interface instead of creating their own logic to
>>>> isolate untrusted device DMAs initiated by userspace.
>>>>
>>>> This proposal describes the uAPI of /dev/ioasid and also sample sequences
>>>> with VFIO as example in typical usages. The driver-facing kernel API provided
>>>> by the iommu layer is still TBD, which can be discussed after consensus is
>>>> made on this uAPI.
>>>>
>>>> It's based on a lengthy discussion starting from here:
>>>> https://lore.kernel.org/linux-iommu/20210330132830.GO2356281@nvidia.com/
>>>>
>>>> It ends up to be a long writing due to many things to be summarized and
>>>> non-trivial effort required to connect them into a complete proposal.
>>>> Hope it provides a clean base to converge.
>>>>
>>> [..]
>>>
>>>> /*
>>>>    * 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
>>>>    */
>>> Hi,
>>>
>>> It seems that the ioasid has different usage in different situation, it could
>>> be directly used in the physical routing, or just a virtual handle that indicates
>>> a page table or a vPASID table (such as the GPA address space, in the simple
>>> passthrough case, the DMA input to IOMMU will just contain a Stream ID, no
>>> Substream ID), right?
>>>
>>> And Baolu suggested that since one device might consume multiple page tables,
>>> it's more reasonable to have one fault handler per page table. By this, do we
>>> have to maintain such an ioasid info list in the IOMMU layer?
>> As discussed earlier, the I/O page fault and cache invalidation paths
>> will have "device labels" so that the information could be easily
>> translated and routed.
>>
>> So it's likely the per-device fault handler registering API in iommu
>> core can be kept, but /dev/ioasid will be grown with a layer to
>> translate and propagate I/O page fault information to the right
>> consumers.
> Yeah, having a general preprocessing of the faults in IOASID seems to be
> a doable direction. But since there may be more than one consumer at the
> same time, who is responsible for registering the per-device fault handler?

The drivers register per page table fault handlers to /dev/ioasid which
will then register itself to iommu core to listen and route the per-
device I/O page faults. This is just a top level thought. Haven't gone
through the details yet. Need to wait and see what /dev/ioasid finally
looks like.

Best regards,
baolu