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

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Message-ID: <Z4TokbA1s3OyNdjt@macbook.local>
Date: Mon, 13 Jan 2025 11:18:57 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: Bjorn Helgaas <helgaas@...nel.org>
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 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 also wonder whether Xen and Linux (or guest OSes in general) would
agree on how to reference such devices.  AFAICT VMD segment numbers
are purely a software construct, but not something enforced by the
specification.  Could for example FreeBSD assign a different segment
to VMD devices?

If so we would need some kind of specification about how VMD segment
values are assigned so that OSes have a coherent way of referencing
VMD devices without ambiguity.

Thanks, Roger.