Re: [PATCH 02/11] driver core: Set DMA ownership during driver bind/unbind

[Jason Gunthorpe]

On Mon, Nov 15, 2021 at 03:37:18PM +0000, Robin Murphy wrote:

> IOMMUs, and possibly even fewer of them support VFIO, so I'm in full
> agreement with Greg and Christoph that this absolutely warrants being scoped
> per-bus. I mean, we literally already have infrastructure to prevent drivers
> binding if the IOMMU/DMA configuration is broken or not ready yet; why would
> we want a totally different mechanism to prevent driver binding when the
> only difference is that that configuration *is* ready and working to the
> point that someone's already claimed it for other purposes?

I see, that does make sense

I see these implementations:

drivers/amba/bus.c:     .dma_configure  = platform_dma_configure,
drivers/base/platform.c:        .dma_configure  = platform_dma_configure,
drivers/bus/fsl-mc/fsl-mc-bus.c:        .dma_configure  = fsl_mc_dma_configure,
drivers/pci/pci-driver.c:       .dma_configure  = pci_dma_configure,
drivers/gpu/host1x/bus.c:       .dma_configure = host1x_dma_configure,

Other than host1x they all work with VFIO.

Also, there is no bus->dma_unconfigure() which would be needed to
restore the device as well.

So, would you rather see duplicated code into the 4 drivers, and a new
bus op to 'unconfigure dma'

Or, a 'dev_configure_dma()' function that is roughly:

        if (dev->bus->dma_configure) {
                ret = dev->bus->dma_configure(dev);
                if (ret)
                        return ret;
                if (!drv->suppress_auto_claim_dma_owner) {
                       ret = iommu_device_set_dma_owner(dev, DMA_OWNER_KERNEL,
                                                        NULL);
                       if (ret)
                               ret;
                }
         }

And a pair'd undo.

This is nice because we can enforce dev->bus->dma_configure when doing
a user bind so everything holds together safely without relying on
each bus_type to properly implement security.

Jason