Re: [PATCH v2] PCI: vmd: Enable Hotplug based on BIOS setting on VMD rootports

From: Bjorn Helgaas <helgaas@kernel.org>
To: Nirmal Patel <nirmal.patel@linux.intel.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>,
	linux-pci@vger.kernel.org,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Kai-Heng Feng <kai.heng.feng@canonical.com>
Subject: Re: [PATCH v2] PCI: vmd: Enable Hotplug based on BIOS setting on VMD rootports
Date: Wed, 7 Feb 2024 12:55:49 -0600	[thread overview]
Message-ID: <20240207185549.GA910460@bhelgaas> (raw)
In-Reply-To: <a296e02527c6465edcb051d2393e2e6e612a1b0d.camel@linux.intel.com>

On Tue, Feb 06, 2024 at 05:27:29PM -0700, Nirmal Patel wrote:
> ...
> Did you have a chance to look at my response on January 16th to your
> questions? I tried to summarize all of the potential problems and
> issues with different fixes. Please let me know if it is easier if I
> resend the explanation. Thanks.

I did see your Jan 16 response, thanks.

I had more questions after reading it, but they're more about
understanding the topology seen by the host and the guest:
  Jan 16: https://lore.kernel.org/r/20240117004933.GA108810@bhelgaas
  Feb  1: https://lore.kernel.org/r/20240201211620.GA650432@bhelgaas

As I mentioned in my second Feb 1 response
(https://lore.kernel.org/r/20240201222245.GA650725@bhelgaas), the
usual plan envisioned by the PCI Firmware spec is that an OS can use a
PCIe feature if the platform has granted the OS ownership via _OSC and
a device advertises the feature via a Capability in config space.

My main concern with the v2 patch
(https://lore.kernel.org/r/20231127211729.42668-1-nirmal.patel@linux.intel.com)
is that it overrides _OSC for native_pcie_hotplug, but not for any of
the other features (AER, PME, LTR, DPC, etc.)

I think it's hard to read the specs and conclude that PCIe hotplug is
a special case, and I think we're likely to have similar issues with
other features in the future.

But if you think this is the best solution, I'm OK with merging it.

Bjorn