RE: [PATCH 2/4 RESEND] dma-mapping: Add wrapper function to set dma_coherent

[Michael Kelley (LINUX)]

From: Robin Murphy <robin.murphy@xxxxxxx> Sent: Thursday, March 17, 2022 10:20 AM
> 
> On 2022-03-17 16:25, Michael Kelley via iommu wrote:
> > Add a wrapper function to set dma_coherent, avoiding the need for
> > complex #ifdef's when setting it in architecture independent code.
> 
> No. It might happen to work out on the architectures you're looking at,
> but if Hyper-V were ever to support, say, AArch32 VMs you might see the
> problem. arch_setup_dma_ops() is the tool for this job.
>

OK.   There's currently no vIOMMU in a Hyper-V guest, so presumably the
code would call arch_setup_dma_ops() with the dma_base, size, and iommu
parameters set to 0 and NULL.  This call can then be used in Patch 3 instead
of acpi_dma_configure(), and in the Patch 4 hv_dma_configure() function
as you suggested.  arch_setup_dma_ops() is not exported, so I'd need to
wrap it in a Hyper-V specific function in built-in code that is exported.

But at some point in the future if there's a vIOMMU in Hyper-V guests,
this approach will need some rework.

Does that make sense?  Thanks for your input and suggestions ...

Michael