Re: [Qemu-devel] A question about PCI device address spaces

[Peter Xu <peterx@redhat.com>]

On Fri, Dec 23, 2016 at 11:21:53AM +0000, Peter Maydell wrote:
> On 22 December 2016 at 09:42, Peter Xu <peterx@redhat.com> wrote:
> > Hello,
> >
> > Since this is a general topic, I picked it out from the VT-d
> > discussion and put it here, just want to be more clear of it.
> >
> > The issue is, whether we have exposed too much address spaces for
> > emulated PCI devices?
> >
> > Now for each PCI device, we are having PCIDevice::bus_master_as for
> > the device visible address space, which derived from
> > pci_device_iommu_address_space():
> >
> > AddressSpace *pci_device_iommu_address_space(PCIDevice *dev)
> > {
> >     PCIBus *bus = PCI_BUS(dev->bus);
> >     PCIBus *iommu_bus = bus;
> >
> >     while(iommu_bus && !iommu_bus->iommu_fn && iommu_bus->parent_dev) {
> >         iommu_bus = PCI_BUS(iommu_bus->parent_dev->bus);
> >     }
> >     if (iommu_bus && iommu_bus->iommu_fn) {
> >         return iommu_bus->iommu_fn(bus, iommu_bus->iommu_opaque, dev->devfn);
> >     }
> >     return &address_space_memory;
> > }
> >
> > By default (for no-iommu case), it's pointed to system memory space,
> > which includes MMIO, and looks wrong - PCI device should not be able to
> > write to MMIO regions.
> 
> This is just legacy, I think, ie a combination of "this used to
> be system memory space so let's not break things" and "PC works
> mostly like this". It should be possible for the PCI host bridge
> emulation to set things up so that the device's visible address
> space is whatever it feels like. The PCI APIs we have for doing
> this have "iommu" in the name but they work just as well even
> if the host bridge doesn't actually have an iommu and is just
> setting up a fixed or slightly configurable mapping.
> I think it just hasn't been implemented because for guests which
> aren't misbehaving it doesn't make any difference.

Hmm, yes I see ppc e500 is using that for setting up its own address
space, possibly x86 can leverage it too when we really need this. For
now, I see no strong reason for this enhancement, so let me keep it as
it is, and wait until we have both a strong reason and a PCI guru.

Thank you for your answer! (to Paolo/David as well)

-- peterx