linux-kernel - Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Date:   Wed, 31 Mar 2021 21:37:05 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

On Wed, Mar 31, 2021 at 04:46:21PM -0700, Jacob Pan wrote:
> Hi Jason,
> 
> On Wed, 31 Mar 2021 09:38:01 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > > > Get rid of the ioasid set.
> > > >
> > > > Each driver has its own list of allowed ioasids.  
> >  [...]  
> > 
> > The /dev/ioasid FD replaces this security check. By becoming FD
> > centric you don't need additional kernel security objects.
> > 
> > Any process with access to the /dev/ioasid FD is allowed to control
> > those PASID. The seperation between VMs falls naturally from the
> > seperation of FDs without creating additional, complicated, security
> > infrastrucure in the kernel.
> > 
> > This is why all APIs must be FD focused, and you need to have a
> > logical layering of responsibility.
> > 
> >  Allocate a /dev/ioasid FD
> >  Allocate PASIDs inside the FD
> >  Assign memory to the PASIDS
> > 
> >  Open a device FD, eg from VFIO or VDP
> >  Instruct the device FD to authorize the device to access PASID A in
> >  an ioasid FD
> How do we know user provided PASID A was allocated by the ioasid FD?

You pass in the ioasid FD and use a 'get pasid from fdno' API to
extract the required kernel structure.

> Shouldn't we validate user input by tracking which PASIDs are
> allocated by which ioasid FD?

Yes, but it is integral to the ioasid FD, not something separated.

> > VFIO extracts some kernel representation of the ioasid from the ioasid
> > fd using an API
> > 
> This lookup API seems to be asking for per ioasid FD storage array. Today,
> the ioasid_set is per mm and contains a Xarray. 

Right, put the xarray per FD. A set per mm is fairly nonsensical, we
don't use the mm as that kind of security key.

> Since each VM, KVM can only open one ioasid FD, this per FD array
> would be equivalent to the per mm ioasid_set, right?

Why only one?  Each interaction with the other FDs should include the
PASID/FD pair. There is no restriction to just one.

> > VFIO does some kernel call to IOMMU/IOASID layer that says 'tell the
> > IOMMU that this PCI device is allowed to use this PASID'
>
> Would it be redundant to what iommu_uapi_sva_bind_gpasid() does? I thought
> the idea is to use ioasid FD IOCTL to issue IOMMU uAPI calls. Or we can
> skip this step for now and wait for the user to do SVA bind.

I'm not sure what you are asking.

Possibly some of the IOMMU API will need a bit adjusting to make
things split.

The act of programming the page tables and the act of authorizing a
PCI BDF to use a PASID are distinct things with two different IOCTLs.

iommu_uapi_sva_bind_gpasid() is never called by anything, and it's
uAPI is never implemented.

Joerg? Why did you merge dead uapi and dead code?

Jason