(2012/10/03 22:23), Don Dutile wrote: > On 10/02/2012 03:49 AM, Takao Indoh wrote: >> These patches reset PCIe devices at boot time to address DMA problem on >> kdump with iommu. When "reset_devices" is specified, a hot reset is >> triggered on each PCIe root port and downstream port to reset its >> downstream endpoint. >> >> Background: >> A kdump problem about DMA has been discussed for a long time. That is, >> when a kernel is switched to the kdump kernel DMA derived from first >> kernel affects second kernel. Recently this problem surfaces when iommu >> is used for PCI passthrough on KVM guest. In the case of the machine I >> use, when intel_iommu=on is specified, DMAR error is detected in kdump >> kernel and PCI SERR is also detected. Finally kdump fails because some >> devices does not work correctly. >> >> The root cause is that ongoing DMA from first kernel causes DMAR fault >> because page table of DMAR is initialized while kdump kernel is booting >> up. Therefore to address this problem DMA needs to be stopped before >> DMAR is initialized at kdump kernel boot time. By these patches, PCIe >> devices are reset by hot reset and its DMA is stopped when reset_devices >> is specified. One problem of this solution is that the monitor blacks >> out when VGA controller is reset. So this patch does not reset the port >> whose child endpoint is VGA device. >> >> v2: >> Reset devices in setup_arch() because reset need to be done before >> interrupt remapping is initialized. >> >> v1: >> https://lkml.org/lkml/2012/8/3/160 >> >> Thanks, >> Takao Indoh >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-pci" in >> the body of a message to majordomo at vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Maybe you've tried the following, and I missed a thread on it, > but instead of a somewhat-large, reset hammer, did any one try > just reading all the endpoint-only device CMD register, flip the MasterEnable > bit off, and write it back? .... that would stop all DMA (should stop > all MSI writes as well since they are just another DMA), and then > restart the system? > May also have to do PCI INT Disable as well... and note, that's a PCI 2.3 > optional feature.... so devices using INT signalling vs MSI > is just borked on IOMMU/intr-remapping systems... which I would expect > are few. Then again, if this is foolishly done, then reset legacy PCI > busses as the fallback. Just clearing bus master bit and INTx disable bit in setup_arch() did not solve this problem. I still got DMAR error on devices(for exmaple, igb and megaraid_sas). Clearing bus master in setup_arch() and resetting devices in fixup_final like v1 patch is better, DMAR error was not detected. But on certain machine kdump kernel hung up when resetting devices. It seems to be a problem specific to the platform. And, resetting devices in setup_arch() like v2 patch solves all problems I found so far. Thanks, Takao Indoh
