Re: [PATCH] PCI: vmd: Enable Hotplug based on BIOS setting on VMD rootports

[Nirmal Patel <nirmal.patel@linux.intel.com>]

On Wed, 2023-11-01 at 17:20 -0500, Bjorn Helgaas wrote:
> On Tue, Oct 31, 2023 at 12:59:34PM -0700, Nirmal Patel wrote:
> > On Tue, 2023-10-31 at 10:31 -0500, Bjorn Helgaas wrote:
> > > On Mon, Oct 30, 2023 at 04:16:54PM -0400, Nirmal Patel wrote:
> > > > VMD Hotplug should be enabled or disabled based on VMD
> > > > rootports'
> > > > Hotplug configuration in BIOS. is_hotplug_bridge is set on each
> > > > VMD rootport based on Hotplug capable bit in SltCap in probe.c.
> > > > Check is_hotplug_bridge and enable or disable
> > > > native_pcie_hotplug
> > > > based on that value.
> > > > 
> > > > Currently VMD driver copies ACPI settings or platform
> > > > configurations
> > > > for Hotplug, AER, DPC, PM, etc and enables or disables these
> > > > features
> > > > on VMD bridge which is not correct in case of Hotplug.
> > > 
> > > This needs some background about why it's correct to copy the
> > > ACPI
> > > settings in the case of AER, DPC, PM, etc, but incorrect for
> > > hotplug.
> > > 
> > > > Also during the Guest boot up, ACPI settings along with VMD
> > > > UEFI
> > > > driver are not present in Guest BIOS which results in assigning
> > > > default values to Hotplug, AER, DPC, etc. As a result Hotplug
> > > > is
> > > > disabled on VMD in the Guest OS.
> > > > 
> > > > This patch will make sure that Hotplug is enabled properly in
> > > > Host
> > > > as well as in VM.
> > > 
> > > Did we come to some consensus about how or whether _OSC for the
> > > host
> > > bridge above the VMD device should apply to devices in the
> > > separate
> > > domain below the VMD?
> > 
> > We are not able to come to any consensus. Someone suggested to copy
> > either all _OSC flags or none. But logic behind that assumption is
> > that the VMD is a bridge device which is not completely true. VMD
> > is an
> > endpoint device and it owns its domain.
> 
> Do you want to facilitate a discussion in the PCI firmware SIG about
> this?  It seems like we may want a little text in the spec about how
> to handle this situation so platforms and OSes have the same
> expectations.
The patch 04b12ef163d1 broke intel VMD's hotplug capabilities and
author did not test in VM environment impact.
We can resolve the issue easily by 
#1 Revert the patch which means restoring VMD's original functionality
and author provide better fix.

or

#2 Allow the current change to re-enable VMD hotplug inside VMD driver.

There is a significant impact for our customers hotplug use cases which
forces us to apply the fix in out-of-box drivers for different OSs.

nirmal
> 
> Bjorn