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: Wed, 06 Mar 2024 15:27:20 -0700	[thread overview]
Message-ID: <46b737db266f08c6dc98c77044bab12491a4d971.camel@linux.intel.com> (raw)
In-Reply-To: <af071f45513778a9392efb1a9f41f1e18d2670f0.camel@linux.intel.com>

On Tue, 2024-02-13 at 10:47 -0700, Nirmal Patel wrote:
> 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.
Hi Bjorn,

We tried some other ways to pass the information about all of the flags
but I couldn't retrieve it in guest OS and VMD hardware doesn't have
empty registers to write this information. So as of now we have this
solution which only overwrites Hotplug flag. If you can accept it that
would be great.

Thanks
nirmal.

> 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