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

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Robin Murphy <robin.murphy@arm.com> Sent: Friday, March 18, 2022 4:08 AM
> 
> 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!)

Agreed!

> 
> Thanks,
> Robin.