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

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Date:   Tue, 1 Jun 2021 13:10:34 +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

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.

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