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

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Date:   Tue, 1 Jun 2021 14:24:59 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Liu Yi L <yi.l.liu@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        David Gibson <david@...son.dropbear.id.au>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Tue, Jun 01, 2021 at 11:08:53AM +0800, Lu Baolu wrote:
> On 6/1/21 2:09 AM, Jason Gunthorpe wrote:
> > > > device bind should fail if the device somehow isn't compatible with
> > > > the scheme the user is tring to use.
> > > yeah, I guess you mean to fail the device attach when the IOASID is a
> > > nesting IOASID but the device is behind an iommu without nesting support.
> > > right?
> > Right..
> 
> Just want to confirm...
> 
> Does this mean that we only support hardware nesting and don't want to
> have soft nesting (shadowed page table in kernel) in IOASID?

No, the uAPI presents a contract, if the kernel can fulfill the
contract then it should be supported.

If you want SW nesting then the kernel has to have the SW support for
it or fail.

At least for the purposes of document I wouldn't devle too much deeper
into that question.

Jason