Re: [RFC] /dev/ioasid uAPI proposal

[Shenming Lu <lushenming@huawei.com>]

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?

Thanks,
Shenming

> 
> If things evolve in this way, probably the SVA I/O page fault also needs
> to be ported to /dev/ioasid.
> 
>>
>> Then if we add host IOPF support (for the GPA address space) in the future
>> (I have sent a series for this but it aimed for VFIO, I will convert it for
>> IOASID later [1] :-)), how could we find the handler for the received fault
>> event which only contains a Stream ID... Do we also have to maintain a
>> dev(vPASID)->ioasid mapping in the IOMMU layer?
>>
>> [1] https://lore.kernel.org/patchwork/cover/1410223/
> 
> Best regards,
> baolu
> .