linux-kernel - Re: [PATCH v3] PCI: vmd: Honor ACPI

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Date:   Wed, 9 Feb 2022 15:36:38 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Keith Busch <kbusch@...nel.org>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Nirmal Patel <nirmal.patel@...ux.intel.com>,
        Jonathan Derrick <jonathan.derrick@...ux.dev>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] PCI: vmd: Honor ACPI _OSC on PCIe features

On Tue, Dec 07, 2021 at 02:15:04PM +0100, Rafael J. Wysocki wrote:
> On Tue, Dec 7, 2021 at 12:12 AM Keith Busch <kbusch@...nel.org> wrote:
> > On Fri, Dec 03, 2021 at 11:15:41AM +0800, Kai-Heng Feng wrote:
> > > When Samsung PCIe Gen4 NVMe is connected to Intel ADL VMD, the
> > > combination causes AER message flood and drags the system performance
> > > down.
> > >
> > > The issue doesn't happen when VMD mode is disabled in BIOS, since AER
> > > isn't enabled by acpi_pci_root_create() . When VMD mode is enabled, AER
> > > is enabled regardless of _OSC:
> > > [    0.410076] acpi PNP0A08:00: _OSC: platform does not support [AER]
> > > ...
> > > [    1.486704] pcieport 10000:e0:06.0: AER: enabled with IRQ 146
> > >
> > > Since VMD is an aperture to regular PCIe root ports, honor ACPI _OSC to
> > > disable PCIe features accordingly to resolve the issue.
> >
> > At least for some versions of this hardare, I recall ACPI is unaware of
> > any devices in the VMD domain; the platform can not see past the VMD
> > endpoint, so I throught the driver was supposed to always let the VMD
> > domain use OS native support regardless of the parent's ACPI _OSC.
> 
> This is orthogonal to whether or not ACPI is aware of the VMD domain
> or the devices in it.
> 
> If the platform firmware does not allow the OS to control specific
> PCIe features at the physical host bridge level, that extends to the
> VMD "bus", because it is just a way to expose a hidden part of the
> PCIe hierarchy.

I don't understand what's going on here.  Do we understand the AER
message flood?  Are we just papering over it by disabling AER?

If an error occurs below a VMD, who notices and reports it?  If we
disable native AER below VMD because of _OSC, as this patch does, I
guess we're assuming the platform will handle AER events below VMD.
Is that really true?  Does the platform know how to find AER log
registers of devices below VMD?

> The platform firmware does that through ACPI _OSC under the host
> bridge device (not under the VMD device) which it is very well aware
> of.