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

[Jonathan Derrick <jonathan.derrick@linux.dev>]

On 2/9/2022 2:36 PM, Bjorn Helgaas 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?
> 
> 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?
ACPI (and the specific UEFI implementation) might remain unaware of
VMD domains. It's possible that the system management mode (SMM)
controller which typically handles firmware-first errors would be
capable of handling VMD errors in the vendor-specific manner.
However if _OSC hadn't taken into account VMD ports, SMM wouldn't
be capable of handling those errors and silently disabling AER on
VMD domains is a bad idea.

The bugzilla made it sound like a specific platform/drive combination.
What about a DMI match to mask the Corrected Physical Layer bits?

> 
>> 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.