Re: [PATCH v3] PCI: vmd: Honor ACPI _OSC on PCIe features

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Feb 15, 2022 at 4:09 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Mon, Feb 14, 2022 at 08:23:05AM +0800, Kai-Heng Feng wrote:
> > On Thu, Feb 10, 2022 at 5:36 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > 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@kernel.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?
> >
> > To be more precise, AER is disabled by the platform vendor in BIOS to
> > paper over the issue.
> > The only viable solution for us is to follow their settings. We may
> > never know what really happens underneath.
> >
> > Disabling ASPM/AER/PME etc is a normal practice for ODMs unfortunately.
>
> OK.  So this patch actually has nothing in particular to do with AER.
> It's about making _OSC apply to *all* devices below a host bridge,
> even those below a VMD.

Right.

> This is slightly ambiguous because while "_OSC applies to the entire
> hierarchy originated by a PCI Host Bridge" (PCI Firmware spec r3.3,
> sec 4.5.1), vmd.c creates a logical view where devices below the VMD
> are in a separate hierarchy with a separate domain.

But from the HW perspective they still are in the same hierarchy below
the original host bridge.

> The interpretation that _OSC applies to devices below VMD should work,
> as long as it is possible for platform firmware to manage services
> (AER, pciehp, etc) for things below VMD without getting in the way of
> vmd.c.

vmd.c actually exposes things hidden by the firmware and the point of
the patch is to still let the firmware control them if it wants/needs
to IIUC.

> But I think one implication of this is that we cannot support
> hot-added VMDs.  For example, firmware that wants to manage AER will
> use _OSC to retain AER control.  But if the firmware doesn't know how
> VMDs work, it will not be able to handle AER for devices below the
> VMD.

Well, the firmware needs to know how stuff works to hide it in the
first place ...

> > > 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.