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Date: Tue, 12 Aug 2025 14:00:36 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: Nam Cao <namcao@linutronix.de>
Cc: Keith Busch <kbusch@kernel.org>,
	Nirmal Patel <nirmal.patel@linux.intel.com>,
	Jonathan Derrick <jonathan.derrick@linux.dev>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	Krzysztof =?utf-8?Q?Wilczy=C2=B4nski?= <kwilczynski@kernel.org>,
	Manivannan Sadhasivam <mani@kernel.org>,
	Rob Herring <robh@kernel.org>, Bjorn Helgaas <bhelgaas@google.com>,
	Thomas Gleixner <tglx@linutronix.de>, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org, Ammar Faizi <ammarfaizi2@gnuweeb.org>
Subject: Re: [PATCH] PCI: vmd: Remove MSI-X check on child devices
Message-ID: <20250812190036.GA199875@bhelgaas>
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In-Reply-To: <20250812182209.c31roKpC@linutronix.de>

On Tue, Aug 12, 2025 at 08:22:09PM +0200, Nam Cao wrote:
> 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".

Thanks, updated.