Re: [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdumpwith iommu

[Takao Indoh]

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(-)
>
>
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>
>
>
> --
> ------------------
> Oliver Yang

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