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SubjectRe: [PATCH 1/3] x86/quirks: Scan all busses for early PCI quirks
[+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 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.

Bjorn

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