Re: [RFC] /dev/ioasid uAPI proposal

[Shenming Lu <lushenming@huawei.com>]

On 2021/6/1 20:30, Lu Baolu wrote:
> 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.

OK. And it needs to be confirmed by Jean since we might migrate the code from
io-pgfault.c to IOASID... Anyway, finalize /dev/ioasid first.  Thanks,

Shenming

> 
> Best regards,
> baolu
> .