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

[Jason Gunthorpe]

On Mon, Nov 15, 2021 at 06:35:37PM +0000, Robin Murphy wrote:
> 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.

The group will be on the default_domain regardless if a kernel driver
is bound or not, so the number of bound kernel drivers still needs to
be tracked and restored.

> > 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.

bus specific variation does not fill me with confidence - there should
not be bus specific variation on security principles, especially when
the API is supporting VFIO and the like. 

How can we reason about that?

Jason