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

From: Takao Indoh
Date: Tue Sep 11 2012 - 06:37:47 EST

(2012/09/10 23:36), Vivek Goyal wrote:
On Wed, Sep 05, 2012 at 08:09:58PM +0900, Takao Indoh wrote:
(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

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.

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

(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().

I would not claim that I understand hte PCI SERR issue. But whatever
resettings needs to happen, should happen early in second kernel.

Doing it in first kernel is not a good idea as it is crashed kernel and
we want to as little as possible.

Doing it in purgatory is not a good idea either as purgatory does not
konw anything about kernel as such. We don't want to bloat purgatory
with reset code and embedding the device knowledge there.

Keeping it in second kernel makes sense so that code remains with kernel
and can be maintained there.

I'll post new patch which clears bus master bit and resets devices in
second kernel.

As to the boot parameter to enable this function, you suggested using
reset_devices. I found that on a certain platform resetting devices
caused PCIe error due to a hardware bug. Therefore I think we need
new parameter apart from reset_devices to disable this function on
such a machine.

Takao Indoh

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