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

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Date:   Wed, 2 Jun 2021 13:07:08 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.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>,
        "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>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Wed, Jun 02, 2021 at 04:52:02PM +0800, Jason Wang wrote:
> 
> 在 2021/6/2 上午4:28, Jason Gunthorpe 写道:
> > > I summarized five opens here, about:
> > > 
> > > 1)  Finalizing the name to replace /dev/ioasid;
> > > 2)  Whether one device is allowed to bind to multiple IOASID fd's;
> > > 3)  Carry device information in invalidation/fault reporting uAPI;
> > > 4)  What should/could be specified when allocating an IOASID;
> > > 5)  The protocol between vfio group and kvm;
> > > 
> > > For 1), two alternative names are mentioned: /dev/iommu and
> > > /dev/ioas. I don't have a strong preference and would like to hear
> > > votes from all stakeholders. /dev/iommu is slightly better imho for
> > > two reasons. First, per AMD's presentation in last KVM forum they
> > > implement vIOMMU in hardware thus need to support user-managed
> > > domains. An iommu uAPI notation might make more sense moving
> > > forward. Second, it makes later uAPI naming easier as 'IOASID' can
> > > be always put as an object, e.g. IOMMU_ALLOC_IOASID instead of
> > > IOASID_ALLOC_IOASID.:)
> > I think two years ago I suggested /dev/iommu and it didn't go very far
> > at the time.
> 
> 
> It looks to me using "/dev/iommu" excludes the possibility of implementing
> IOASID in a device specific way (e.g through the co-operation with device
> MMU + platform IOMMU)?

This is intended to be the 'drivers/iommu' subsystem though. I don't
want to see pluggabilit here beyoned what drivers/iommu is providing.

If someone wants to do a something complicated through this interface
then they need to make a drivers/iommu implementation.

Or they need to use the mdev-esque "SW TABLE" mode when their driver
attaches to the interface.

If this is good enough or not for a specific device is an entirely
other question though

> What's more, ATS spec doesn't forbid the device #PF to be reported via a
> device specific way.

And if this is done then a kernel function indicating page fault
should be triggered on the ioasid handle that the device has. It is
still drivers/iommu functionality

Jason