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

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Date:   Thu, 1 Apr 2021 14:26:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, "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 Thu, Apr 01, 2021 at 10:23:55AM -0700, Jacob Pan wrote:
> Hi Jason,
> 
> On Wed, 31 Mar 2021 21:37:05 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > 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
> Just to be super clear. Do we allocate a FD for each PASID and return the
> FD to the user? Or return the plain PASID number back to the user space?

I would do multiple PASID's per /dev/ioasid FD because we expect alot
of PASIDs to be in use and we'd run into FDno limits.

> > > >  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.
> > 
> Seems you are talking about two FDs:
> - /dev/ioasid FD

No, just this one.

> - per IOASID FD
> This API ioasid = get_pasid_from_fd(dev_ioasid_fd, ioasid_fd);
> dev_ioasid_fd will find the xarray for all the PASIDs allocated under it,
> ioasid_fd wil be the index into the xarray to retrieve the actual ioasid.
> Correct?

'ioasid_fd' is just the ioasid number in whatever numberspace the
/dev/ioasid FD's use.

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

> OK, one per subsystem-VM. For example, if a VM has a VFIO and a VDPA
> device, it should only two /dev/ioasid FDs respectively. Correct?

No, only one.

For something like qemu's use case I mostly expect the vIOMMU driver
will open /dev/ioasid for each vIOMMU instance it creates (basically
only one)

> > 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.
> > 
> Why separate? 

Because they have different owners and different layers in the
software.

It is not about use case, it is about putting the control points where
they naturally belong.

> For a complex stack like vSVA, I feel we have to reduce moving parts and do
> some divide and conquer.

uAPI should have all come together with a user and user application.

uAPI is hardest and most important part.

Jason