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

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Date:   Tue, 1 Jun 2021 05:42:58 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Wang <jasowang@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "Liu Yi L" <yi.l.liu@...ux.intel.com>
CC:     "Alex Williamson (alex.williamson@...hat.com)\"\"" 
        <alex.williamson@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "Jason Gunthorpe" <jgg@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: RE: [RFC] /dev/ioasid uAPI proposal

> From: Jason Wang
> Sent: Tuesday, June 1, 2021 1:30 PM
> 
> 在 2021/6/1 下午1:23, Lu Baolu 写道:
> > 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.
> 
> 
> Right, that's exactly my question. The latter seems much more easier to
> be understood and implemented.
> 

A simple reason discussed in previous thread - there could be 1M's 
I/O address spaces per device while #FD's are precious resource.
So this RFC treats fd as a container of address spaces which is each
tagged by an IOASID.

Thanks
Kevin