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

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Message-ID: <20250812182209.c31roKpC@linutronix.de>
Date: Tue, 12 Aug 2025 20:22:09 +0200
From: Nam Cao <namcao@...utronix.de>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Keith Busch <kbusch@...nel.org>,
	Nirmal Patel <nirmal.patel@...ux.intel.com>,
	Jonathan Derrick <jonathan.derrick@...ux.dev>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczy´nski <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 Tue, Aug 12, 2025 at 11:30:15AM -0500, Bjorn Helgaas wrote:
> On Tue, Aug 12, 2025 at 08:49:50AM -0600, Keith Busch wrote:
> > The doc you linked is riddled with errors. The original vmd commit
> > message is more accurate: VMD domains support child devices with MSI and
> > MSI-x interrupts. The VMD device can't even tell the difference which
> > one the device is using. It just manipulates messages sent to the usual
> > APIC address 0xfeeXXXXX.
> 
> Thanks, Keith!  I updated the commit log like this:
> 
>   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].
> 
>   However, the VMD device can't even tell the difference between a child
>   device using MSI and one using MSI-X.  Per 185a383ada2e ("x86/PCI: Add
>   driver for Intel Volume Management Device (VMD)"), VMD does not support
>   INTx interrupts, but does support child devices using either MSI or MSI-X.
> 
>   Remove the sanity check to avoid the unnecessary WARN_ON reported by Ammar.

Minor correction, it is not just an unnecessary WARN_ON, but child devices'
drivers couldn't enable MSI at all.

So perhaps something like "Remove the sanity check to allow child devices
which only support MSI".

Thank you both,
Nam