On 10/02/2014 08:09 AM, Bjorn Helgaas wrote: > On Tue, Sep 30, 2014 at 12:15 AM, Li, ZhenHua <zhen-hual@xxxxxx> wrote: >> Add Joerg to CC list. For it is also related to iommu module. >> >> Joerg, >> There was a try for this dmar fault, >> https://lkml.org/lkml/2014/8/18/118 >> >> This patch is trying to fix the same thing. >> >> >> Zhenhua >> >> On 09/30/2014 02:09 PM, Li, Zhen-Hua wrote: >>> On a HP system with Intel Corporation 82599 ethernet adapter, when kernel >>> crashed and the kdump kernel boots with intel_iommu=on, there may be some >>> unexpected DMA requests on this adapter, which will cause DMA Remapping >>> faults like: >>> dmar: DRHD: handling fault status reg 102 >>> dmar: DMAR:[DMA Read] Request device [41:00.0] fault addr fff81000 >>> DMAR:[fault reason 01] Present bit in root entry is clear >>> >>> Analysis for this bug: >>> >>> The present bit is set in this function: >>> >>> static struct context_entry * device_to_context_entry( >>> struct intel_iommu *iommu, u8 bus, u8 devfn) >>> { >>> ...... >>> set_root_present(root); >>> ...... >>> } >>> >>> Calling tree: >>> ixgbe_open >>> ixgbe_setup_tx_resources >>> intel_alloc_coherent >>> __intel_map_single >>> domain_context_mapping >>> domain_context_mapping_one >>> device_to_context_entry >>> >>> This means, the present bit in root entry will not be set until the device >>> driver is loaded. >>> >>> But in the kdump kernel, some hardware device does not know the OS is the >>> second kernel and the drivers should be loaded again, this causes there >>> are >>> some unexpected DMA requsts on this device when it has not been >>> initialized, >>> and then the DMA Remapping errors come. >>> >>> To fix this DMAR fault, we need to reset the bus that this device on. >>> Reset >>> the device itself does not work. > This seems like something that could happen with *any* device, not > just the 82599 NIC. Or is there something in the "kernel crash -> > kexec -> kdump kernel" path that stops DMA for most devices, but not > for the 82599?lex > This is an *any* device problem. Specifically any device that is doing active DMA when a kdump kernel is triggered will cause this issue since the IOMMU will not have valid mappings for the DMA events until the device driver itself is loaded and resets the device. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
