RE: [RFC][PATCH] Reset PCIe devices to address DMA problem on kdumpwith iommu

[Kaneshige, Kenji]

> -----Original Message-----
> From: linux-pci-owner@xxxxxxxxxxxxxxx
> [mailto:linux-pci-owner@xxxxxxxxxxxxxxx] On Behalf Of Takao Indoh
> Sent: Wednesday, September 05, 2012 8:10 PM
> To: vgoyal@xxxxxxxxxx
> Cc: kexec@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> linux-pci@xxxxxxxxxxxxxxx; bhelgaas@xxxxxxxxxx; hbabu@xxxxxxxxxx; Ishii,
> Hironobu/石井 宏延; martin.wilck@xxxxxxxxxxxxxx
> 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.

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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