linux-kernel - RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user

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Date:   Mon, 18 Oct 2021 01:52:14 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Lu Baolu <baolu.lu@...ux.intel.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>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Friday, October 15, 2021 7:10 PM
> 
> On Fri, Oct 15, 2021 at 01:29:16AM +0000, Tian, Kevin wrote:
> > Hi, Jason,
> >
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, September 29, 2021 8:59 PM
> > >
> > > On Wed, Sep 29, 2021 at 12:38:35AM +0000, Tian, Kevin wrote:
> > >
> > > > /* If set the driver must call iommu_XX as the first action in probe() or
> > > >   * before it attempts to do DMA
> > > >   */
> > > >  bool suppress_dma_owner:1;
> > >
> > > It is not "attempts to do DMA" but more "operates the physical device
> > > in any away"
> > >
> > > Not having ownership means another entity could be using user space
> > > DMA to manipulate the device state and attack the integrity of the
> > > kernel's programming of the device.
> > >
> >
> > Does suppress_kernel_dma sounds better than suppress_dma_owner?
> > We found the latter causing some confusion when doing internal
> > code review. Somehow this flag represents "don't claim the kernel dma
> > ownership during driver binding". suppress_dma_owner sounds the
> > entire ownership is disabled...
> 
> If in doubt make it
> 
> suppress_iommu_whatever_the_api_is_that_isn't_called

ok

> 
> > Another thing is about DMA_OWNER_SHARED, which is set to indicate
> > no dma at all. Thinking more we feel that this flag is meaningless. Its
> > sole purpose is to show compatibility to any USER/KERNEL ownership,
> > and essentially the same semantics as a device which is not bound to
> > any driver. So we plan to remove it then pci-stub just needs one line
> > change to set the suppress flag. But want to check with you first in case
> > any oversight.
> 
> It sounds reasonable, but also makes it much harder to find the few
> places that have this special relationship - ie we can't grep for
> DMA_OWNER_SHARED anymore.
> 

It's probably fine. People can just search the suppress flag and filter out
drivers with DMA_OWNER_USER. Then the remaining set is about 
drivers in SHARED category. Less straightforward but should be fine
for a relatively small set of drivers.

Thanks
Kevin