linux-kernel - Re: [PATCH] PCI: vmd: Remove MSI-X check on child devices

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Message-ID: <20250812062715.X9TWWWh-@linutronix.de>
Date: Tue, 12 Aug 2025 08:27:15 +0200
From: Nam Cao <namcao@...utronix.de>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Nirmal Patel <nirmal.patel@...ux.intel.com>,
	Jonathan Derrick <jonathan.derrick@...ux.dev>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kwilczynski@...nel.org>,
	Manivannan Sadhasivam <mani@...nel.org>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ammar Faizi <ammarfaizi2@...weeb.org>
Subject: Re: [PATCH] PCI: vmd: Remove MSI-X check on child devices

On Mon, Aug 11, 2025 at 05:46:59PM -0500, Bjorn Helgaas wrote:
> On Mon, Aug 11, 2025 at 07:39:35AM +0200, Nam Cao wrote:
> > Commit d7d8ab87e3e7 ("PCI: vmd: Switch to msi_create_parent_irq_domain()")
> > added a WARN_ON sanity check that child devices support MSI-X, because VMD
> > document says [1]:
> > 
> >     "Intel VMD only supports MSIx Interrupts from child devices and
> >     therefore the BIOS must enable PCIe Hot Plug and MSIx interrups [sic]."
> 
> Can VMD tell the difference between an incoming MSI MWr transaction
> and an MSI-X MWr?
> 
> I wonder if "MSIx" was meant to mean "VMD only supports MSI or MSI-X
> interrupts, not INTx interrupts, from child devices"?

I don't know, sorry. I am hoping that the VMD maintainers can help us here.

But I don't expect to get an answer soon, and this is affecting users, so I
prefer to merge it regardless.

And to be honest, depending on the answer, this patch may be the wrong fix.
But in this worst case, we would just bring the driver back to how it had
always behaved, and no one complained before. Therefore it should be safe to
do.

Nam