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Date: Fri, 21 Dec 2012 19:37:03 +0900
From: Takao Indoh <indou.takao@...fujitsu.com>
To: yangoliver@...il.com
CC: linux-pci@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org, andi@...stfloor.org,
tokunaga.keiich@...fujitsu.com, kexec@...ts.infradead.org,
hbabu@...ibm.com, mingo@...hat.com, ddutile@...hat.com,
vgoyal@...hat.com, ishii.hironobu@...fujitsu.com, hpa@...or.com,
bhelgaas@...gle.com, tglx@...utronix.de, yinghai@...nel.org,
khalid@...ehiking.org
Subject: Re: [PATCH v7 0/5] Reset PCIe devices to address DMA problem on kdump
with iommu
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@...fujitsu.com <mailto:indou.takao@...fujitsu.com>>
>
> 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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