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

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Stuart Yoder <stuyoder@gmail.com>,
	rafael@kernel.org, David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
	Ashok Raj <ashok.raj@intel.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Christoph Hellwig <hch@infradead.org>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	kvm@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v4 02/13] driver core: Set DMA ownership during driver bind/unbind
Date: Thu, 23 Dec 2021 10:08:11 +0800	[thread overview]
Message-ID: <40d67b07-b56b-d626-f71b-54ef5c80275c@linux.intel.com> (raw)
In-Reply-To: <YcMeZlN3798noycN@kroah.com>

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
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