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SubjectRe: [PATCH 1/3] x86/quirks: Scan all busses for early PCI quirks
Bjorn Helgaas <helgaas@kernel.org> writes:

> [+cc Rafael for question about ACPI method for PCI host bridge reset]
>
> On Sat, Nov 14, 2020 at 09:58:08PM +0100, Thomas Gleixner wrote:
>> On Sat, Nov 14 2020 at 14:39, Bjorn Helgaas wrote:
>> > On Sat, Nov 14, 2020 at 12:40:10AM +0100, Thomas Gleixner wrote:
>> >> On Sat, Nov 14 2020 at 00:31, Thomas Gleixner wrote:
>> >> > On Fri, Nov 13 2020 at 10:46, Bjorn Helgaas wrote:
>> >> >> pci_device_shutdown() still clears the Bus Master Enable bit if we're
>> >> >> doing a kexec and the device is in D0-D3hot, which should also disable
>> >> >> MSI/MSI-X. Why doesn't this solve the problem? Is this because the
>> >> >> device causing the storm was in PCI_UNKNOWN state?
>> >> >
>> >> > That's indeed a really good question.
>> >>
>> >> So we do that on kexec, but is that true when starting a kdump kernel
>> >> from a kernel crash? I doubt it.
>> >
>> > Ah, right, I bet that's it, thanks. The kdump path is basically this:
>> >
>> > crash_kexec
>> > machine_kexec
>> >
>> > while the usual kexec path is:
>> >
>> > kernel_kexec
>> > kernel_restart_prepare
>> > device_shutdown
>> > while (!list_empty(&devices_kset->list))
>> > dev->bus->shutdown
>> > pci_device_shutdown # pci_bus_type.shutdown
>> > machine_kexec
>> >
>> > So maybe we need to explore doing some or all of device_shutdown() in
>> > the crash_kexec() path as well as in the kernel_kexec() path.
>>
>> The problem is that if the machine crashed anything you try to attempt
>> before starting the crash kernel is reducing the chance that the crash
>> kernel actually starts.
>
> Right.
>
>> Is there something at the root bridge level which allows to tell the
>> underlying busses to shut up, reset or go into a defined state? That
>> might avoid chasing lists which might be already unreliable.
>
> Maybe we need some kind of crash_device_shutdown() that does the
> minimal thing to protect the kdump kernel from devices.

The kdump kernel does not use any memory the original kernel uses.
Which should be a minimal and fairly robust level of protection
until the device drivers can be loaded and get ahold of things.

> The programming model for conventional PCI host bridges and PCIe Root
> Complexes is device-specific since they're outside the PCI domain.
> There probably *are* ways to do those things, but you would need a
> native host bridge driver or something like an ACPI method. I'm not
> aware of an ACPI way to do this, but I added Rafael in case he is.
>
> A crash_device_shutdown() could do something at the host bridge level
> if that's possible, or reset/disable bus mastering/disable MSI/etc on
> individual PCI devices if necessary.

Unless I am confused DMA'ing to memory that is not already in use
is completely broken wether or not you are using the kdump kernel.

Eric

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