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

[Robin Murphy <robin.murphy@arm.com>]

On 2022-03-17 19:13, Michael Kelley (LINUX) wrote:
> From: Robin Murphy <robin.murphy@arm.com> 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 ...

Yes, that's essentially what I had in mind. If you did present a vIOMMU 
to the guest, presumably you'd either have to construct a regular 
IORT/VIOT, and thus end up adding the root complex to the ACPI namespace 
too so it can be referenced, at which point it would all get picked up 
by the standard machinery, or come up with some magic VMBus mechanism 
that would need a whole load of work to wire up in all the relevant 
places anyway.

(But please lean extremely heavily towards the former option!)

Thanks,
Robin.