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

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Date:   Tue, 1 Jun 2021 13:23:07 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jason Wang <jasowang@...hat.com>,
        Liu Yi L <yi.l.liu@...ux.intel.com>
Cc:     baolu.lu@...ux.intel.com, yi.l.liu@...el.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>, Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

Hi Jason W,

On 6/1/21 1:08 PM, Jason Wang wrote:
>>> 2) If yes, what's the reason for not simply use the fd opened from
>>> /dev/ioas. (This is the question that is not answered) and what happens
>>> if we call GET_INFO for the ioasid_fd?
>>> 3) If not, how GET_INFO work?
>> oh, missed this question in prior reply. Personally, no special reason
>> yet. But using ID may give us opportunity to customize the management
>> of the handle. For one, better lookup efficiency by using xarray to
>> store the allocated IDs. For two, could categorize the allocated IDs
>> (parent or nested). GET_INFO just works with an input FD and an ID.
> 
> 
> I'm not sure I get this, for nesting cases you can still make the child 
> an fd.
> 
> And a question still, under what case we need to create multiple ioasids 
> on a single ioasid fd?

One possible situation where multiple IOASIDs per FD could be used is
that devices with different underlying IOMMU capabilities are sharing a
single FD. In this case, only devices with consistent underlying IOMMU
capabilities could be put in an IOASID and multiple IOASIDs per FD could
be applied.

Though, I still not sure about "multiple IOASID per-FD" vs "multiple
IOASID FDs" for such case.

Best regards,
baolu