Re: [PATCH v2 0/2] Reset PCIe devices to address DMA problem on kdump with iommu

Takao Indoh
2012-10-09T09:08:33Z

(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