On 2021-11-15 15:56, Jason Gunthorpe via iommu wrote:
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.
Not if we reduce the notion of "ownership" down to
"dev->iommu_group->domain != dev->iommu_group->default_domain", which
I'm becoming increasingly convinced is all we actually need here.
So, would you rather see duplicated code into the 4 drivers, and a new
bus op to 'unconfigure dma'
The .dma_configure flow is unavoidably a bit boilerplatey already, so
personally I'd go for having the implementations call back into a common
check, similarly to their current flow. That also leaves room for the
bus code to further refine the outcome based on what it might know,
which I can particularly imagine for cleverer buses like fsl-mc and
host1x which can have lots of inside knowledge about how their devices
may interact.
Robin.
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
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