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

From: Takao Indoh
Date: Fri Dec 21 2012 - 05:37:34 EST


(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.

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.

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.
This PCI SERR seems to be related to interrupt remapping.
- Clearing bus master in setup_arch() and resetting devices in
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.

Update Yinghai's dummy-pci patch with macros in linux/pci.h, and fix
some bugs

Rewrite using Yinghai's dummy-pci patch

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.

Reduce waiting time after resetting devices. A previous patch does reset
like this:
for (each device) {
save config registers
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
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.

Move alloc_bootmem and free_bootmem to early_reset_pcie_devices so that
they are called only once.

Reset devices in setup_arch() because reset need to be done before
interrupt remapping is initialized.

Add fixup_final quirk to reset PCIe devices

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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