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

From: ebiederm@xmission.com (Eric W. Biederman)
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	"Guilherme G. Piccoli" <gpiccoli@canonical.com>,
	linux-pci@vger.kernel.org, kexec@lists.infradead.org,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	bhelgaas@google.com, dyoung@redhat.com, bhe@redhat.com,
	vgoyal@redhat.com, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
	andi@firstfloor.org, lukas@wunner.de, okaya@kernel.org,
	kernelfans@gmail.com, ddstreet@canonical.com,
	gavin.guo@canonical.com, jay.vosburgh@canonical.com,
	kernel@gpiccoli.net, shan.gavin@linux.alibaba.com,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH 1/3] x86/quirks: Scan all busses for early PCI quirks
Date: Sun, 15 Nov 2020 08:05:05 -0600	[thread overview]
Message-ID: <87v9e6n2b2.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20201114212215.GA1194074@bjorn-Precision-5520> (Bjorn Helgaas's message of "Sat, 14 Nov 2020 15:22:15 -0600")

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