Re: issues with emulated PCI MMIO backed by host memory under KVM

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jun 27, 2016 at 02:15:29PM +0100, Peter Maydell wrote:
> On 27 June 2016 at 10:47, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > As for the USB case, I can't really figure out what is going on here,
> > but I am fairly certain it is a different issue. If this is related to
> > DMA, I wonder if adding the 'dma-coherent' property to the PCIe root
> > complex node fixes anything.
> 
> I get the impression dma-coherent is the right thing to advertise
> anyway. Do you have the documentation to hand that specifies what
> "dma-coherent" means? The Documentation/devicetree docs in the
> kernel tree seem to rather unhelpfully define it as "Present if
> dma operations are coherent", which doesn't really clarify anything
> to me...

It's ill-defined today, and the precise definition is an open question.
See replies to [1], which seems to have stalled as of [2].

My view is that for arm/arm64 this should mean the device makes accesses
which are coherent with Inner Shareable Normal Inner-WB Outer-WB
attributes, as this is the functional de-facto semantics today, and
anything short of that is not well-defined or usable.

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-June/433626.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-June/434143.html