On 22/02/17 23:39, Bjorn Helgaas wrote: > [+cc Joerg, iommu list] > > On Wed, Feb 22, 2017 at 03:44:53PM -0500, Sinan Kaya wrote: >> On 2/22/2017 1:44 PM, Bjorn Helgaas wrote: >>> There is no way for a driver to say "I only need this memory BAR and >>> not the other ones." The reason is because the PCI_COMMAND_MEMORY bit >>> enables *all* the memory BARs; there's no way to enable memory BARs >>> selectively. If we enable memory BARs and one of them is unassigned, >>> that unassigned BAR is enabled, and the device will respond at >>> whatever address the register happens to contain, and that may cause >>> conflicts. >>> >>> I'm not sure this answers your question. Do you want to get rid of >>> 32-bit BAR addresses because your host bridge doesn't have a window to >>> 32-bit PCI addresses? It's typical for a bridge to support a window >>> to the 32-bit PCI space as well as one to the 64-bit PCI space. Often >>> it performs address translation for the 32-bit window so it doesn't >>> have to be in the 32-bit area on the CPU side, e.g., you could have >>> something like this where we have three host bridges and the 2-4GB >>> space on each PCI root bus is addressable: >>> >>> pci_bus 0000:00: root bus resource [mem 0x1080000000-0x10ffffffff] (bus address [0x80000000-0xffffffff]) >>> pci_bus 0001:00: root bus resource [mem 0x1180000000-0x11ffffffff] (bus address [0x80000000-0xffffffff]) >>> pci_bus 0002:00: root bus resource [mem 0x1280000000-0x12ffffffff] (bus address [0x80000000-0xffffffff]) >> >> The problem is that according to PCI specification BAR addresses and >> DMA addresses cannot overlap. >> >> From PCI-to-PCI Bridge Arch. spec.: "A bridge forwards PCI memory >> transactions from its primary interface to its secondary interface >> (downstream) if a memory address is in the range defined by the >> Memory Base and Memory Limit registers (when the base is less than >> or equal to the limit) as illustrated in Figure 4-3. Conversely, a >> memory transaction on the secondary interface that is within this >> address range will not be forwarded upstream to the primary >> interface." >> >> To be specific, if your DMA address happens to be in >> [0x80000000-0xffffffff] and root port's aperture includes this >> range; the DMA will never make to the system memory. >> >> Lorenzo and Robin took some steps to carve out PCI addresses out of >> DMA addresses in IOMMU drivers by using iova_reserve_pci_windows() >> function. >> >> However, I see that we are still exposed when the operating system >> doesn't have any IOMMU driver and is using the SWIOTLB for instance. > > Hmmm. I guess SWIOTLB assumes there's no address translation in the > DMA direction, right? Not entirely - it does rely on arch-provided dma_to_phys() and phys_to_dma() helpers which are free to accommodate such translations in a device-specific manner. On arm64 we use these to account for dev->dma_pfn_offset describing a straightforward linear offset, but unless one constant offset would apply to all possible outbound windows I'm not sure that's much help here. > If there's no address translation in the PIO > direction, PCI bus BAR addresses are identical to the CPU-side > addresses. In that case, there's no conflict because we already have > to assign BARs so they never look like a system memory address. > > But if there *is* address translation in the PIO direction, we can > have conflicts because the bridge can translate CPU-side PIO accesses > to arbitrary PCI bus addresses. > >> The FW solution I'm looking at requires carving out some part of the >> DDR from before OS boot so that OS doesn't reclaim that area for >> DMA. > > If you want to reach system RAM, I guess you need to make sure you > only DMA to bus addresses outside the host bridge windows, as you said > above. DMA inside the windows would be handled as peer-to-peer DMA. > >> I'm not very happy with this solution. I'm also surprised that there >> is no generic solution in the kernel takes care of this for all root >> ports regardless of IOMMU driver presence. > > The PCI core isn't really involved in allocating DMA addresses, > although there definitely is the connection with PCI-to-PCI bridge > windows that you mentioned. I added IOMMU guys, who would know a lot > more than I do. To me, having the bus addresses of windows shadow assigned physical addresses sounds mostly like a broken system configuration. Can the firmware not reprogram them elsewhere, or is the entire bottom 4GB of the physical memory map occupied by system RAM? Robin. > > Bjorn > _______________________________________________ > iommu mailing list > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx > https://lists.linuxfoundation.org/mailman/listinfo/iommu >
