On 10/09/2012 05:03 AM, Takao Indoh wrote:
(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
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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
this summary should be in the patch set, so others know/learn
what was attempted, what failed, and how you reached this working conclusion.
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