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

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Date:   Fri, 15 Oct 2021 08:09:56 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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

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

> 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.

Jason