linux-kernel - Re: [PATCH 2/2] PCI: Disable PCIE hotplug interrupts early when msi is disabled

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Message-ID: <Z6RU-681eXl7hcp6@wunner.de>
Date: Thu, 6 Feb 2025 07:21:47 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Feng Tang <feng.tang@...ux.alibaba.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Jonathan Cameron <Jonthan.Cameron@...wei.com>,
	ilpo.jarvinen@...ux.intel.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] PCI: Disable PCIE hotplug interrupts early when msi
 is disabled

[to += Rafael, start of thread is here:
https://lore.kernel.org/all/Z6HcoUB3i51bzQDs@wunner.de/
]

Hi Rafael,

On Wed, Feb 05, 2025 at 11:58:04AM +0800, Feng Tang wrote:
> On Tue, Feb 04, 2025 at 10:23:45AM +0100, Lukas Wunner wrote:
> > On Tue, Feb 04, 2025 at 01:37:58PM +0800, Feng Tang wrote:
> > > There was a irq storm bug when testing "pci=nomsi" case, and the root
> > > cause is: 'nomsi' will disable MSI and let devices and root ports use
> > > legacy INTX inerrupt, and likely make several devices/ports share one
> > > interrupt. In the failure case, BIOS doesn't disable the PCIE hotplug
> > > interrupts, and  actually asserts the command-complete interrupt.
> > > As MSI is disabled, ACPI initialization code will not enumerate root
> > > port's PCIE hotplug capability, and pciehp service driver wont' be
> > > enabled for the root port to handle that interrupt, later on when it is
> > > shared and enabled by other device driver like NVME or NIC, the "nobody
> > > care irq storm" happens.
> >
> > Is there a section in the PCI Firmware Spec which says ACPI doesn't
> > enumerate the hotplug capability if MSI is disabled?
> 
> No, I didn't get it from spec, but found the logic by code reading
> during debugging the irq storm issue. The related code is about:
> 
> #define ACPI_PCIE_REQ_SUPPORT (OSC_PCI_EXT_CONFIG_SUPPORT \
> 				| OSC_PCI_ASPM_SUPPORT \
> 				| OSC_PCI_CLOCK_PM_SUPPORT \
> 				| OSC_PCI_MSI_SUPPORT)

Commit 415e12b23792 ("PCI/ACPI: Request _OSC control once for each root
bridge (v3)") contains a change which doesn't seem to be explained in
the commit message:

If the user passes "pci=nomsi" on the command line, Linux doesn't
request hotplug control (or any other control) from the platform.
So ACPI always remains responsible for hotplug in the "pci=nomsi"
case.

The commit sought to fix a cpu hog issue:

https://bugzilla.kernel.org/show_bug.cgi?id=29722

It's unclear to me if that bug was fixed by requesting _OSC only once,
as the commit message suggests, or if the addition of OSC_MSI_SUPPORT
to ACPI_PCIE_REQ_SUPPORT fixed the issue.

Since the latter is not mentioned in the commit message,
it seems plausible to assume that the OSC_MSI_SUPPORT change
was unintentional.

In any case it doesn't seem to make sense to not request any
control in the "pci=nomsi" case.

It's also worth noting that the behavior is different on
Apple machines as they use a fixed _OSC set even for "pci=nomsi".

I'm wondering if OSC_PCI_MSI_SUPPORT should simply be removed
from ACPI_PCIE_REQ_SUPPORT, but I'm worried that it may cause
reappearance of the cpu hog issue.

Thoughts?

Thanks,

Lukas