linux-kernel - Re: [PATCH 1/3] x86/quirks: Scan all busses for early PCI quirks

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Message-ID: <20201114212215.GA1194074@bjorn-Precision-5520>
Date:   Sat, 14 Nov 2020 15:22:15 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Guilherme G. Piccoli" <gpiccoli@...onical.com>,
        linux-pci@...r.kernel.org, kexec@...ts.infradead.org,
        x86@...nel.org, linux-kernel@...r.kernel.org, bhelgaas@...gle.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, andi@...stfloor.org,
        lukas@...ner.de, okaya@...nel.org, kernelfans@...il.com,
        ddstreet@...onical.com, gavin.guo@...onical.com,
        jay.vosburgh@...onical.com, kernel@...ccoli.net,
        shan.gavin@...ux.alibaba.com,
        Eric Biederman <ebiederm@...ssion.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [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