linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Message-ID: <01fe5034-42c8-6923-32f1-e287cc36bccc@linux.intel.com>
Date:   Tue, 1 Jun 2021 20:30:35 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Shenming Lu <lushenming@...wei.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, Jason Wang <jasowang@...hat.com>
Cc:     baolu.lu@...ux.intel.com, Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        "wanghaibin.wang@...wei.com" <wanghaibin.wang@...wei.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

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