Re: [Qemu-devel] [PATCH v2 3/4] arm: Add PCIe host bridge in virt machine

[Peter Maydell]

On 29 January 2015 at 14:37, Alexander Graf <address@hidden> wrote:
> On 29.01.15 15:34, Peter Maydell wrote:
>> I kind of see, but isn't this just a window from CPU address
>> space into PCI address space, not vice-versa?
>
> Yup, exactly. But PCI devices need to map themselves somewhere into the
> PCI address space. So if I configure a BAR to live at 0x10000000, it
> should also show up at 0x10000000 when accessed from the CPU. That's
> what the mapping above is about.

No, it doesn't have to. It's a choice to make the mapping
be such that the system address for a BAR matches the address
in PCI memory space, not a requirement. I agree it's a
sensible choice, though.

But as I say, this code is setting up one mapping (the
system address -> PCI space mapping), not two.

>> DMA by PCI devices bus-mastering into system memory must be
>> being set up elsewhere, I think.
>
> Yes, that's a different mechanism that's not implemented yet for GPEX
> :).

We can't not implement DMA, it would break lots of the usual
PCI devices people want to use. In fact I thought the PCI
core code implemented a default of "DMA by PCI devices goes
to the system address space" if you didn't specifically
set up something else by calling pci_setup_iommu(). This is
definitely how it works for plain PCI host bridges, are
PCIe bridges different?

> On ARM this would happen via SMMU emulation.

There's no requirement for a PCI host controller to be
sat behind an SMMU -- that's a system design choice. We
don't need to implement the SMMU yet (or perhaps ever?);
we definitely need to support PCI DMA.

-- PMM