linux-kernel - Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Message-ID: <b0b5b9b3-083e-1c77-276d-d6eef84806a6@linux.intel.com>
Date:   Mon, 10 May 2021 10:56:37 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     baolu.lu@...ux.intel.com, Jason Gunthorpe <jgg@...dia.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs

On 5/8/21 3:31 PM, Tian, Kevin wrote:
>> From: Alex Williamson<alex.williamson@...hat.com>
>> Sent: Saturday, May 8, 2021 1:06 AM
>>
>>>> Those are the main ones I can think of.  It is nice to have a simple
>>>> map/unmap interface, I'd hope that a new /dev/ioasid interface wouldn't
>>>> raise the barrier to entry too high, but the user needs to have the
>>>> ability to have more control of their mappings and locked page
>>>> accounting should probably be offloaded somewhere.  Thanks,
>>>>
>>> Based on your feedbacks I feel it's probably reasonable to start with
>>> a type1v2 semantics for the new interface. Locked accounting could
>>> also start with the same VFIO restriction and then improve it
>>> incrementally, if a cleaner way is intrusive (if not affecting uAPI).
>>> But I didn't get the suggestion on "more control of their mappings".
>>> Can you elaborate?
>> Things like I note above, userspace cannot currently specify mapping
>> granularity nor has any visibility to the granularity they get from the
>> IOMMU.  What actually happens in the IOMMU is pretty opaque to the user
>> currently.  Thanks,
>>
> It's much clearer. Based on all the discussions so far I'm thinking about
> a staging approach when building the new interface, basically following
> the model that Jason pointed out - generic stuff first, then platform
> specific extension:
> 
> Phase 1: /dev/ioasid with core ingredients and vfio type1v2 semantics
>      - ioasid is the software handle representing an I/O page table

A trivial proposal, is it possible to use /dev/ioas? Conceptually, it's
an IO address space representation and has nothing to do with any ID.

Best regards,
baolu