netdev - Re: [PATCH iwl-net v2] ice: reset first in crash dump kernels

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Message-ID: <791.1696355031@famine>
Date: Tue, 03 Oct 2023 10:43:51 -0700
From: Jay Vosburgh <jay.vosburgh@...onical.com>
To: Jesse Brandeburg <jesse.brandeburg@...el.com>
cc: intel-wired-lan@...ts.osuosl.org, linux-pci@...r.kernel.org,
    pmenzel@...gen.mpg.de, netdev@...r.kernel.org, jkc@...hat.com,
    "Vishal
 Agrawal" <vagrawal@...hat.com>,
    Przemek Kitszel <przemyslaw.kitszel@...el.com>
Subject: Re: [PATCH iwl-net v2] ice: reset first in crash dump kernels

Jesse Brandeburg <jesse.brandeburg@...el.com> wrote:

>On 10/2/2023 4:49 PM, Jay Vosburgh wrote:
>> Jesse Brandeburg <jesse.brandeburg@...el.com> wrote:
>> 
>>> When the system boots into the crash dump kernel after a panic, the ice
>>> networking device may still have pending transactions that can cause errors
>>> or machine checks when the device is re-enabled. This can prevent the crash
>>> dump kernel from loading the driver or collecting the crash data.
>>>
>>> To avoid this issue, perform a function level reset (FLR) on the ice device
>>> via PCIe config space before enabling it on the crash kernel. This will
>>> clear any outstanding transactions and stop all queues and interrupts.
>>> Restore the config space after the FLR, otherwise it was found in testing
>>> that the driver wouldn't load successfully.
>> 
>> 	How does this differ from ading "reset_devices" to the crash
>> kernel command line, per Documentation/admin-guide/kdump/kdump.rst?
>> 
>> 	-J
>> 
>
>Hi Jay, thanks for the question.
>
>That parameter is new to me, and upon looking into the parameter, it
>doesn't seem well documented. It also seems to only be used by storage
>controllers, and would basically result in the same code I already have.
>I suspect since it's a driver opt-in to the parameter, the difference
>would be 1) requiring the user to give the reset_devices parameter on
>the kdump kernel line (which is a big "if") and 2) less readable code
>than the current which does:
>
>if (is_kdump_kernel())
>...
>
>and the reset_devices way would be:
>
>if (reset_devices)
>...
>
>There are several other examples in the networking tree using the method
>I ended up with in this change. I'd argue the preferred way in the
>networking tree is to use is_kdump_kernel(), which I like better because
>it doesn't require user input and shouldn't have any bad side effects
>from doing an extra reset in kdump.
>
>Also, this issue has already been tested to be fixed by this patch.
>
>I'd prefer to keep the patch as is, if that's ok with you.

	Thanks for the explanation; I was wondering if this methodology
would conflict or compete with reset_devices in some way, or if there's
a risk that the FLR would in some cases make things worse.

	Since many device drivers have this sort of logic in them, would
it make sense to put this in the PCI core somewhere to FLR at probe time
if is_kdump_kernel()?  The manifestation of the issue that I'm familiar
with is that DMA requests from the device arrive after the IOMMU DMA
remapping tables have been reset during kexec, leading to failures.

	Regardless, the patch looks fine to me given the current state
of kdump / kexec / reset_devices.

Reviewed-by: Jay Vosburgh <jay.vosburgh@...onical.com>

	-J

---
	-Jay Vosburgh, jay.vosburgh@...onical.com