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

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Message-ID: <20250113232926.GA442589@bhelgaas>
Date: Mon, 13 Jan 2025 17:29:26 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Roger Pau Monné <roger.pau@...rix.com>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH 1/3] xen/pci: do not register devices outside of PCI
 segment scope

On Mon, Jan 13, 2025 at 11:18:57AM +0100, Roger Pau Monné wrote:
> On Fri, Jan 10, 2025 at 04:21:29PM -0600, Bjorn Helgaas wrote:
> > On Fri, Jan 10, 2025 at 03:01:48PM +0100, 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.
> > 
> > The ACPI _SEG method (ACPI r6.5, sec 6.5.6) and the corresponding
> > value in the MCFG table (PCI Firmware r3.3, sec 4.1.2) are clearly
> > 16-bit values.
> > 
> > But otherwise, the segment value is pretty much an arbitrary software
> > value, and the kernel works fine with the larger domain values from
> > vmd_find_free_domain(), so this isn't quite enough to explain what the
> > issue with Xen is.
> > 
> > Does Xen truncate the domain to 16 bits or use it to look up something
> > in ACPI?
> 
> In the interface between Xen and Linux the segment field is 16 bit
> width, so with the current interface is not possible to reference
> devices that are past the 0xffff segment.

I think this specific reason (and maybe even struct
physdev_pci_device_add) would be more useful than the ACPI _SEG and
MCFG things, which are not as directly connected here.

Bjorn