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

From: Nirmal Patel <nirmal.patel@linux.intel.com>
To: Bjorn Helgaas <helgaas@kernel.org>
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: Tue, 13 Feb 2024 10:47:27 -0700	[thread overview]
Message-ID: <af071f45513778a9392efb1a9f41f1e18d2670f0.camel@linux.intel.com> (raw)
In-Reply-To: <20240207185549.GA910460@bhelgaas>

On Wed, 2024-02-07 at 12:55 -0600, Bjorn Helgaas wrote:
> 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.
I sincerely apologize for late responses. I just found out that my
evolution mail client is automatically sending linux-pci emails to junk
since January 2024.

At the moment Hotplug is an exception for us, but I do share your
concern about other flags. We have done lot of testing and so far patch
v2 is the best solution we have.

Thanks
nirmal
> 
> Bjorn