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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
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Subject: RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces
Thread-Topic: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces
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Date:   Tue, 28 Sep 2021 07:13:01 +0000
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> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Monday, September 27, 2021 10:40 PM
>=20
> On Mon, Sep 27, 2021 at 01:32:34PM +0000, Tian, Kevin wrote:
>=20
> > but I'm little worried that even vfio-pci itself cannot be bound now,
> > which implies that all devices in a group which are intended to be
> > used by the user must be bound to vfio-pci in a breath before the
> > user attempts to open any of them, i.e. late-binding and device-
> > hotplug is disallowed after the initial open. I'm not sure how
> > important such an usage would be, but it does cause user-tangible
> > semantics change.
>=20
> Oh, that's bad..
>=20
> I guess your approach is the only way forward, it will have to be
> extensively justified in the commit message for Greg et al.
>=20

Just thought about another alternative. What about having driver
core to call iommu after call_driver_probe()?

call_driver_probe()
	pci_stub_probe()
		iommu_set_dma_mode(dev, DMA_NONE);
iommu_check_dma_mode(dev);

The default dma mode is DMA_KERNEL. Above allows driver to opt
for DMA_NONE or DMA_USER w/o changing the device_driver
structure. Right after probe() is completed, we check whether dma=20
mode of this device is allowed by the iommu core (based on recorded=20
dma mode info of sibling devices in the same group). If not, then fail=20
the binding.

Thanks
Kevin