Re: Segment truncation in multi-segment PCI handling?

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 07/06/2024 8:46 pm, Marek Marczykowski-Górecki wrote:
> Hi,
>
> I've got a new system, and it has two PCI segments:
>
>     0000:00:00.0 Host bridge: Intel Corporation Device 7d14 (rev 04)
>     0000:00:02.0 VGA compatible controller: Intel Corporation Meteor Lake-P [Intel Graphics] (rev 08)
>     ...
>     10000:e0:06.0 System peripheral: Intel Corporation RST VMD Managed Controller
>     10000:e0:06.2 PCI bridge: Intel Corporation Device 7ecb (rev 10)
>     10000:e1:00.0 Non-Volatile memory controller: Phison Electronics Corporation PS5021-E21 PCIe4 NVMe Controller (DRAM-less) (rev 01)
>
> But looks like Xen doesn't handle it correctly:
>
>     (XEN) 0000:e0:06.0: unknown type 0
>     (XEN) 0000:e0:06.2: unknown type 0
>     (XEN) 0000:e1:00.0: unknown type 0
>     ...
>     (XEN) ==== PCI devices ====
>     (XEN) ==== segment 0000 ====
>     (XEN) 0000:e1:00.0 - NULL - node -1 
>     (XEN) 0000:e0:06.2 - NULL - node -1 
>     (XEN) 0000:e0:06.0 - NULL - node -1 
>     (XEN) 0000:2b:00.0 - d0 - node -1  - MSIs < 161 >
>     (XEN) 0000:00:1f.6 - d0 - node -1  - MSIs < 148 >
>     ...
>
> This isn't exactly surprising, since pci_sbdf_t.seg is uint16_t, so
> 0x10000 doesn't fit. OSDev wiki says PCI Express can have 65536 PCI
> Segment Groups, each with 256 bus segments.
>
> Fortunately, I don't need this to work, if I disable VMD in the
> firmware, I get a single segment and everything works fine.
>

This is a known issue.  Works is being done, albeit slowly.

0x10000 is indeed not a spec-compliant PCI segment.  It's something
model specific the Linux VMD driver is doing.

~Andrew