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    SubjectRe: [PATCH 2/8] PCI: Add pci_find_common_upstream_dev()


    On 29/03/18 05:44 AM, Christian König wrote:
    > Am 28.03.2018 um 21:53 schrieb Logan Gunthorpe:
    >>
    >> On 28/03/18 01:44 PM, Christian König wrote:
    >>> Well, isn't that exactly what dma_map_resource() is good for? As far as
    >>> I can see it makes sure IOMMU is aware of the access route and
    >>> translates a CPU address into a PCI Bus address.
    >>> I'm using that with the AMD IOMMU driver and at least there it works
    >>> perfectly fine.
    >> Yes, it would be nice, but no arch has implemented this yet. We are just
    >> lucky in the x86 case because that arch is simple and doesn't need to do
    >> anything for P2P (partially due to the Bus and CPU addresses being the
    >> same). But in the general case, you can't rely on it.
    >
    > Well, that an arch hasn't implemented it doesn't mean that we don't have
    > the right interface to do it.

    Yes, but right now we don't have a performant way to check if we are
    doing P2P or not in the dma_map_X() wrappers. And this is necessary to
    check if the DMA ops in use support it or not. We can't have the
    dma_map_X() functions do the wrong thing because they don't support it yet.

    > Devices integrated in the CPU usually only "claim" to be PCIe devices.
    > In reality their memory request path go directly through the integrated
    > north bridge. The reason for this is simple better throughput/latency.

    These are just more reasons why our patchset restricts to devices behind
    a switch. And more mess for someone to deal with if they need to relax
    that restriction.

    Logan

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