Re: [RFC][PATCH] Reset PCIe devices to address DMA problem on kdump with iommu

[Takao Indoh <indou.takao@jp.fujitsu.com>]

(2012/09/10 11:34), Kaneshige, Kenji wrote:
>> -----Original Message-----
>> From: linux-pci-owner@vger.kernel.org
>> [mailto:linux-pci-owner@vger.kernel.org] On Behalf Of Takao Indoh
>> Sent: Wednesday, September 05, 2012 8:10 PM
>> To: vgoyal@redhat.com
>> Cc: kexec@lists.infradead.org; linux-kernel@vger.kernel.org;
>> linux-pci@vger.kernel.org; bhelgaas@google.com; hbabu@us.ibm.com; Ishii,
>> Hironobu/石井 宏延; martin.wilck@ts.fujitsu.com
>> Subject: Re: [RFC][PATCH] Reset PCIe devices to address DMA problem on kdump
>> with iommu
>>
>> (2012/08/07 5:39), Vivek Goyal wrote:
>>> On Mon, Aug 06, 2012 at 01:30:47PM +0900, Takao Indoh wrote:
>>>> Hi Vivek,
>>>>
>>>> (2012/08/03 20:46), Vivek Goyal wrote:
>>>>> On Fri, Aug 03, 2012 at 08:24:31PM +0900, Takao Indoh wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> This patch adds kernel parameter "reset_pcie_devices" which resets
>> PCIe
>>>>>> devices at boot time to address DMA problem on kdump with iommu. When
>>>>>> this parameter is specified, a hot reset is triggered on each PCIe
>> root
>>>>>> port and downstream port to reset its downstream endpoint.
>>>>>
>>>>> Hi Takao,
>>>>>
>>>>> Why not use existing "reset_devices" parameter instead of introducing
>>>>> a new one?
>>>>
>>>> "reset_devices" is used for each driver to reset their own device, and
>>>> this patch resets all devices forcibly, so I thought they were different
>>>> things.
>>>
>>> Yes reset_devices currently is used for driver to reset its device. I
>>> thought one could very well extend its reach to reset pci express devices
>>> at bus level.
>>>
>>> Having them separate is not going to be much useful from kdump
>>> perspective. We will end up passing both reset_devices and
>>> reset_pcie_devices to second kernel whill lead to bus level reset as well
>>> as device level reset.
>>>
>>> Ideal situation would be that somehow detect that bus level reset has
>> been
>>> done and skip device level reset (assuming bus level reset obviates the
>>> need of device level reset, please correct me if that's not the case).
>>>
>>> After pcie reset, can we store the state in a variable and drivers can
>>> use that variable to check if PCIe level reset was done or not. If yes,
>>> skip device level reset (Assuming driver knows that device is on a
>>> PCIe slot).
>>>
>>> In that case we will not have to introduce new kernel command line, and
>>> also avoid double reset?
>>
>> I found a problem when testing my patch on some machines.
>>
>> Originally there are two problems in kdump kernel when iommu is enabled;
>> DMAR error and PCI SERR. I thought they are fixed by my patch, but I
>> noticed that PCI SERR is still detected after applying the patch. It
>> seems that something happens when Interrupt Remapping is initialized in
>> kdump kernel.
> 
> I'm not sure, but I guess the PCI SERR might be caused as follows.
> 
> - The 1st kernel enables interrupt remapping. The MSI(-X) address and
>    data registers of PCI devices are programmed in remappable format.
> 
> - At the beginning of 2nd kernel, interrupt remapping is still active.
>    And then it is disabled by enable_IR() function for initialization.
> 
> - PCI device generate an interrupt. At this moment, interrupt remapping
>    is not enabled yet. On the other hand, MSI(-X) address and data of this
>    interrupt is in remappable format because those are programmed by 1st
>    kernel. I guess this might be a cause of PCI SERR.
> 
> I guess clearing command register or disabling MSI before interrupt
> remapping initialization in 2nd kernel might solve the PCI SERR problem.

Thank you for your comment. That makes sense.

Though I think clearing bus master bit is enough, do you think I need to clear
all command register bit not only bus master? 

Thanks,
Takao Indoh


> Regards,
> Kenji Kaneshige
> 
> 
>>
>> Therefore resetting devices has to be done before enable_IR() is
>> called. I have three ideas for it.
>>
>>    (i) Resetting devices in 1st kernel(panic kernel)
>>    We can reset devices before jumping into 2nd kernel. Of course it may
>>    be dangerous to scan pci device tree and call PCI functions in panic'd
>>    kernel. Beforehand we need to collect device information so that only
>>    minimal code could run on panic.
>>
>>    (ii) Resetting devices in purgatory
>>    It seems to be be appropriate place to do this, but I'm not sure
>>    where I can save/restore PCI config when resetting devices in
>>    purgatory.
>>
>>    (iii) Resetting devices in 2nd kernel(kdump kernel)
>>    Important point is to do reset before enable_IR() is called as I wrote
>>    above. I think I should add new function to do reset into
>>    arch/x86/pci/early.c and call it in setup_arch like
>>    early_dump_pci_devices() or early_quirks().
>>
>> Any comments?
>>
>> Thanks,
>> Takao Indoh
>>
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>