linux-kernel - Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID

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Date:   Fri, 1 Oct 2021 16:15:12 +1000
From:   "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 11/20] iommu/iommufd: Add IOMMU_IOASID_ALLOC/FREE

On Wed, Sep 22, 2021 at 03:40:25AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Wednesday, September 22, 2021 1:45 AM
> > 
> > On Sun, Sep 19, 2021 at 02:38:39PM +0800, Liu Yi L wrote:
> > > This patch adds IOASID allocation/free interface per iommufd. When
> > > allocating an IOASID, userspace is expected to specify the type and
> > > format information for the target I/O page table.
> > >
> > > This RFC supports only one type (IOMMU_IOASID_TYPE_KERNEL_TYPE1V2),
> > > implying a kernel-managed I/O page table with vfio type1v2 mapping
> > > semantics. For this type the user should specify the addr_width of
> > > the I/O address space and whether the I/O page table is created in
> > > an iommu enfore_snoop format. enforce_snoop must be true at this point,
> > > as the false setting requires additional contract with KVM on handling
> > > WBINVD emulation, which can be added later.
> > >
> > > Userspace is expected to call IOMMU_CHECK_EXTENSION (see next patch)
> > > for what formats can be specified when allocating an IOASID.
> > >
> > > Open:
> > > - Devices on PPC platform currently use a different iommu driver in vfio.
> > >   Per previous discussion they can also use vfio type1v2 as long as there
> > >   is a way to claim a specific iova range from a system-wide address space.
> > >   This requirement doesn't sound PPC specific, as addr_width for pci
> > devices
> > >   can be also represented by a range [0, 2^addr_width-1]. This RFC hasn't
> > >   adopted this design yet. We hope to have formal alignment in v1
> > discussion
> > >   and then decide how to incorporate it in v2.
> > 
> > I think the request was to include a start/end IO address hint when
> > creating the ios. When the kernel creates it then it can return the
> 
> is the hint single-range or could be multiple-ranges?
> 
> > actual geometry including any holes via a query.
> 
> I'd like to see a detail flow from David on how the uAPI works today with
> existing spapr driver and what exact changes he'd like to make on this
> proposed interface. Above info is still insufficient for us to think about the
> right solution.
> 
> > 
> > > - Currently ioasid term has already been used in the kernel
> > (drivers/iommu/
> > >   ioasid.c) to represent the hardware I/O address space ID in the wire. It
> > >   covers both PCI PASID (Process Address Space ID) and ARM SSID (Sub-
> > Stream
> > >   ID). We need find a way to resolve the naming conflict between the
> > hardware
> > >   ID and software handle. One option is to rename the existing ioasid to be
> > >   pasid or ssid, given their full names still sound generic. Appreciate more
> > >   thoughts on this open!
> > 
> > ioas works well here I think. Use ioas_id to refer to the xarray
> > index.
> 
> What about when introducing pasid to this uAPI? Then use ioas_id
> for the xarray index and ioasid to represent pasid/ssid?

This is probably obsoleted by Jason's other comments, but definitely
don't use "ioas_id" and "ioasid" to mean different things.  Having
meaningfully different things distinguished only by an underscore is
not a good idea.

> At this point
> the software handle and hardware id are mixed together thus need
> a clear terminology to differentiate them.
> 
> 
> Thanks
> Kevin
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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