linux-kernel - Re: [PATCH 1/3] xen/pci: do not register devices outside of PCI segment scope

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Message-ID: <Z4TnWiijgBK3fThI@macbook.local>
Date: Mon, 13 Jan 2025 11:13:46 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: Jan Beulich <jbeulich@...e.com>
Cc: Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 1/3] xen/pci: do not register devices outside of PCI
 segment scope

On Mon, Jan 13, 2025 at 08:17:23AM +0100, Jan Beulich wrote:
> On 10.01.2025 15:01, Roger Pau Monne wrote:
> > The PCI segment value is limited to 16 bits, however there are buses like VMD
> > that fake being part of the PCI topology by adding segment with a number
> > outside the scope of the PCI firmware specification range (>= 0x10000). The
> > MCFG ACPI Table "PCI Segment Group Number" field is defined as having a 16 bit
> > width.
> > 
> > Attempting to register or manage those devices with Xen would result in errors
> > at best, or overlaps with existing devices living on the truncated equivalent
> > segment values.
> > 
> > Skip notifying Xen about those devices.
> 
> Hmm, is simply omitting the notification really all it takes? How would Xen
> manage MSI / MSI-X, for example, without knowing of the device? As per the
> BoF on the summit in Prague(?), I continue to think we need partial driver
> logic in Xen for VMD ...

The basic mode of operation of devices behind a VMD bridge is to
reference the interrupts of the bridge device in its MSI(-X) entries,
so the VMD bridge acts as a de-multiplexer and forwards the interrupts
to the device behind the VMD bridge.  See vmd_alloc_irqs() (and
calling context) in the VMD driver for a reference about how this is
setup and operates.  Note also that devices behind a VMD bridge
operate using a different MSI domain, that uses a custom
irq_compose_msi_msg hook.

Thanks, Roger.