linux-kernel - Re: [PATCH v4 02/13] driver core: Set DMA ownership during driver bind/unbind

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Date:   Thu, 23 Dec 2021 10:08:11 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Dan Williams <dan.j.williams@...el.com>, rafael@...nel.org,
        Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Stuart Yoder <stuyoder@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Li Yang <leoyang.li@....com>,
        Dmitry Osipenko <digetx@...il.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 02/13] driver core: Set DMA ownership during driver
 bind/unbind

Hi Greg,

On 12/22/21 8:47 PM, Greg Kroah-Hartman wrote:
> Which one will actually care about the iommu_device_set_dma_owner()
> call?  All of them?  None of them?  Some of them?
> 
> Again, why can't this just happen in the (very few) bus callbacks that
> care about this?  In following patches in this series, you turn off this
> for the pci_dma_configure users, so what is left?  3 odd bus types that
> are not used often.  How well did you test devices of those types with
> this patchset?
> 
> It's fine to have "suppress" fields when they are the minority, but here
> it's a_very_  tiny tiny number of actual devices in a system that will
> ever get the chance to have this check happen for them and trigger,
> right?

Thank you for your comments. Current VFIO implementation supports
devices on pci/platform/amba/fls-mc buses for user-space DMA. So only
those buses need to call iommu_device_set/release_dma_owner() in their
dma_configure/cleanup() callbacks.

The "suppress" field is only for a few device drivers (not devices), for
example,

- vfio-pci, a PCI device driver used to bind to a PCI device so that it
   could be assigned for user-space DMA.

Other similar drivers in drivers/vfio are vfio-fsl-mc, vfio-amba and
vfio-platform. These drivers will call
iommu_device_set/release_dma_owner(DMA_OWNER_USER) explicitly when the
device is assigned to user.

The logic is that on the affected buses (pci/platform/amba/fls-mc),

- for non-vfio drivers, bus dma_configure/cleanup() will automatically
   call iommu_device_set_dma_owner(KERNEL) for the device; [This is the
   majority cases.]

- for vfio drivers, the auto-call will be suppressed, and the vfio
   drivers are supposed to call iommu_device_set_dma_owner(USER) before
   device is assigned to the userspace. [This is the rare case.]

The KERNEL and USER conflict will be detected in
iommu_device_set_dma_owner() with a -EBUSY return value. In that case,
the driver binding or device assignment should be aborted.

Best regards,
baolu