Re: [PATCH v5 4/4] hw/arm/virt: Add PL330 DMA controller and connect with SMMU v3

[Leif Lindholm]

Hi Peter,

On Tue, Sep 07, 2021 at 12:00:45 +0100, Peter Maydell wrote:
> On Thu, 2 Sept 2021 at 09:23, Li, Chunming <Chunming.Li@verisilicon.com> 
> wrote:
> > Eric Auger wrote:
> >> On 9/2/21 8:46 AM, Li, Chunming wrote:
> >>> Eric Auger wrote:
> 
> >>>> Then I think you need to bring a proper motivation behind adding the
> >>>> PL330 in machvirt besides a testing purpose.
> 
> >>>> After this series you would get a single platform device connected to
> >>>> the SMMU, the PL330. What is the actual use case?
> 
> >>> The actual use case is this:
> >>> 1. We will have a SoC which has SMMUv3 connected with our owned platform
> >>>    Video Encoder/Decoder and other IPs
> >>> 2. We plan to use SMMUv3 stage 1 for continuous memory allocation
> >>>    and stage 2 for memory protection
> >>> 3. We are developing our own IP QEMU models now
> >>> 4. These models will be connected with SMMUv3 in QEMU
> >>> 5. We will use the QEMU board to development IP driver and ensure the 
> >>> driver
> >>>    can work well with Linux SMMU and IOMMU framework
> 
> >> I see and I understand your use case for system modeling purpose.
> >>
> >> This raises few questions/comments though.
> >> - supporting platform device protection from the vIOMMU/ARM makes sense
> >> to me globally. But above use case does not justify (to me) the
> >> introduction of PL330 in machvirt because it would be just for testing
> >> purpose. Peter may validate/invalidate though. Instead I think you
> >> should try to illustrate this feature with DMA capable platform devices
> >> such virtio-net and virtio-block sysbus devices as a counterpart of
> >> their PCIe flavour.
> >
> > Thanks for your suggestion. I will try virtio-net and virtio-block sysbus
> > devices in next step.
> > But I hope to keep PL330 because it's not just for testing purpose.
> > It's a good example to show how to connect platform devices with SMMUv3 
> > based
> > on this patch.
> > I assume other developer may have same requirement.
>
> I agree with Eric here that the 'virt' board is not the right place
> to put this PL330. The 'virt' board is not intended as a dumping
> ground or experimental testbed for every possible arm device or
> system configuration -- we try to keep it at least reasonably
> targeted towards the "generic virtual platform or VM" usecase.

Agreed.

> The "sbsa-ref" platform is intended to look more like "real hardware".
> It's possible that "memory mapped hardware that sits behind an SMMU"
> is something that you would see on the sort of system that sbsa-ref
> is trying to model. I've cc'd Leif who would have a better idea about
> that.

I don't know if there's a glovelike fit there, but I agree it's the
closest fit of any platform in QEMU today.

Most DMA on an SBSA system would normally be expected to come from
PCIe, but some have certainly been released with random platform
devices, and it's certainly conceivable that some platform devices
might wants to make use of DMA.

The potential conflict I can see with what this patchset is the use of
a qemu-generated DT to describe the hw to the OS, whereas sbbr-ref
uses ACPI, generated in-guest with a few hints from QEMU, to abstract
it.

It looks to me like the hand-in-glove fit for this use-case would be
an ebbr-ref platform. (Or I guess SystemReady-ES-ref this week.)

But if the ACPI thing isn't a showstopper, I don't particularly mind
carrying an extra peripheral or two in the address space.

/
    Leif