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

[Jan Beulich]

On 10.01.2025 23:21, 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?

One of the involved public interface structs starts like this:

struct physdev_pci_device_add {
    /* IN */
    uint16_t seg;
    uint8_t bus;
    uint8_t devfn;
    ...

So yes, wider segment values would be truncated. Plus, even if they weren't,
there would need to be coordination between Dom0 and Xen on which devices
gets which segment number, since - as you say - the assignment in Linux is
pretty much arbitrary.

Jan