Hi, (2012/12/21 18:59), oliver yang wrote:
Hi Takao, Because I want to config the kdump in our lab, I have some questions about the backgroud of your patch. 1. Will you patch only work while IOMMU is enabled?
No. It always works when "reset_devices" is specified as boot parameter.
2. Looks like kdump doesn't disable DMA and interrupts in the systems, how can Linux make sure second kernel's memory is not corrupted by devices initialized by first kernel?
See purgatory/purgatory.c in kexec_tools. Before jumping into second kernel, a part of memory is checked using sha256 digest. Thanks, Takao Indoh
2012/11/27 Takao Indoh <indou.takao@xxxxxxxxxxxxxx <mailto:indou.takao@xxxxxxxxxxxxxx>> 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. Especially 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 solve 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. What I tried: - Clearing bus master bit and INTx disable bit at boot time This did not solve this problem. I still got DMAR error on devices. - Resetting devices in fixup_final(v1 patch) DMAR error disappeared, but sometimes PCI SERR was detected. This is well explained here. https://lkml.org/lkml/2012/9/9/245 This PCI SERR seems to be related to interrupt remapping. - Clearing bus master in setup_arch() and resetting devices in fixup_final Neither DMAR error nor PCI SERR occurred. But on certain machine kdump kernel hung up when resetting devices. It seems to be a problem specific to the platform. - Resetting devices in setup_arch() (v2 and later patch) This solution solves all problems I found so far. Changelog: v7: Update Yinghai's dummy-pci patch with macros in linux/pci.h, and fix some bugs v6: Rewrite using Yinghai's dummy-pci patch https://lkml.org/lkml/2012/11/13/118 v5: Do bus reset after all devices are scanned and its config registers are saved. This fixes a bug that config register is accessed without delay after reset. https://lkml.org/lkml/2012/10/17/47 v4: Reduce waiting time after resetting devices. A previous patch does reset like this: for (each device) { save config registers reset wait for 500 ms restore config registers } If there are N devices to be reset, it takes N*500 ms. On the other hand, the v4 patch does: for (each device) { save config registers reset } wait 500 ms for (each device) { restore config registers } Though it needs more memory space to save config registers, the waiting time is always 500ms. https://lkml.org/lkml/2012/10/15/49 v3: Move alloc_bootmem and free_bootmem to early_reset_pcie_devices so that they are called only once. https://lkml.org/lkml/2012/10/10/57 v2: Reset devices in setup_arch() because reset need to be done before interrupt remapping is initialized. https://lkml.org/lkml/2012/10/2/54 v1: Add fixup_final quirk to reset PCIe devices https://lkml.org/lkml/2012/8/3/160 Takao Indoh (5): x86, pci: add dummy pci device for early stage PCI: Define the maximum number of PCI function Make reset_devices available at early stage x86, pci: Reset PCIe devices at boot time x86, pci: Enable PCI INTx when MSI is disabled arch/x86/include/asm/pci-direct.h | 3 + arch/x86/kernel/setup.c | 3 + arch/x86/pci/common.c | 4 +- arch/x86/pci/early.c | 315 +++++++++++++++++++++++++++++++++++++ include/linux/pci.h | 2 + init/main.c | 4 +- 6 files changed, 328 insertions(+), 3 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@xxxxxxxxxxxxxxx <mailto:majordomo@xxxxxxxxxxxxxxx> More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ -- ------------------ Oliver Yang
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