xen pci passthrough stops working after xen/x86: fix initial memory balloon target

[James Dingwall <james@dingwall.me.uk>]

Hi,

We have encountered a regression with pci passthrough since the
Ubuntu 6.8.0-91.92 which included this commit:

commit 74287971dbb3fe322bb316afd9e7fb5807e23bee
Author: Roger Pau Monne <roger.pau@citrix.com>
Date:   Wed May 14 10:04:26 2025 +0200

    xen/x86: fix initial memory balloon target

The problem also happens with a 6.19-rc5 build so doesn't seem to be specific
to the Ubuntu kernel.  We deploy an identical disk image to multiple systems
but only one hardware model shows this error and I'm not certain what the
significant difference is.  Logs associated with one of the pci devices:

non-working 6.19-rc5 build:

# grep 01:00.0 6.19.0-061900rc5-generic.log
Jan 15 10:32:21 pci 0000:01:00.0: [8086:1521] type 00 class 0x020000 PCIe Endpoint
Jan 15 10:32:21 pci 0000:01:00.0: BAR 0 [mem 0x81200000-0x812fffff]
Jan 15 10:32:21 pci 0000:01:00.0: BAR 2 [io 0x4020-0x403f]
Jan 15 10:32:21 pci 0000:01:00.0: BAR 3 [mem 0x81404000-0x81407fff]
Jan 15 10:32:21 pci 0000:01:00.0: ROM [mem 0x81380000-0x813fffff pref]
Jan 15 10:32:21 pci 0000:01:00.0: VF BAR 0 [mem 0x4000200000-0x4000203fff 64bit pref]
Jan 15 10:32:21 pci 0000:01:00.0: VF BAR 0 [mem 0x4000200000-0x400021ffff 64bit pref]: contains BAR 0 for 8 VFs
Jan 15 10:32:21 pci 0000:01:00.0: VF BAR 3 [mem 0x4000220000-0x4000223fff 64bit pref]
Jan 15 10:32:21 pci 0000:01:00.0: VF BAR 3 [mem 0x4000220000-0x400023ffff 64bit pref]: contains BAR 3 for 8 VFs
Jan 15 10:32:21 pcifront pci-0: claiming resource 0000:01:00.0/0
Jan 15 10:32:21 pci 0000:01:00.0: BAR 0 [mem 0x81200000-0x812fffff]: can't claim; address conflict with System RAM [mem 0x80000000-0x87ffffff]
Jan 15 10:32:21 pcifront pci-0: Could not claim resource 0000:01:00.0/0! Device offline. Try using e820_host=1 in the guest config.
Jan 15 10:32:21 pcifront pci-0: claiming resource 0000:01:00.0/2
Jan 15 10:32:21 pcifront pci-0: claiming resource 0000:01:00.0/3
Jan 15 10:32:21 pci 0000:01:00.0: BAR 3 [mem 0x81404000-0x81407fff]: can't claim; address conflict with System RAM [mem 0x80000000-0x87ffffff]
Jan 15 10:32:21 pcifront pci-0: Could not claim resource 0000:01:00.0/3! Device offline. Try using e820_host=1 in the guest config.
Jan 15 10:32:21 pcifront pci-0: claiming resource 0000:01:00.0/6
Jan 15 10:32:21 pci 0000:01:00.0: ROM [mem 0x81380000-0x813fffff pref]: can't claim; address conflict with System RAM [mem 0x80000000-0x87ffffff]
Jan 15 10:32:21 pcifront pci-0: Could not claim resource 0000:01:00.0/6! Device offline. Try using e820_host=1 in the guest config.
Jan 15 10:32:21 pcifront pci-0: claiming resource 0000:01:00.0/7
Jan 15 10:32:21 pcifront pci-0: claiming resource 0000:01:00.0/10
Jan 15 10:32:21 igb 0000:01:00.0: BAR 0 [mem size 0x00100000]: not assigned; can't enable device
Jan 15 10:32:21 igb 0000:01:00.0: probe with driver igb failed with error -22


working Ubuntu-6.8.0-100.100 (commit reverted)

# dmesg | grep 01:00.0
6.8.0-100-generic
[ 1.643953] pci 0000:01:00.0: [8086:1521] type 00 class 0x020000 PCIe Endpoint
[ 1.650247] pci 0000:01:00.0: BAR 0 [mem 0x81200000-0x812fffff]
[ 1.650488] pci 0000:01:00.0: BAR 2 [io 0x4020-0x403f]
[ 1.650608] pci 0000:01:00.0: BAR 3 [mem 0x81404000-0x81407fff]
[ 1.650970] pci 0000:01:00.0: ROM [mem 0x81380000-0x813fffff pref]
[ 1.682412] pci 0000:01:00.0: VF BAR 0 [mem 0x4000200000-0x4000203fff 64bit pref]
[ 1.682418] pci 0000:01:00.0: VF BAR 0 [mem 0x4000200000-0x400021ffff 64bit pref]: contains BAR 0 for 8 VFs
[ 1.682773] pci 0000:01:00.0: VF BAR 3 [mem 0x4000220000-0x4000223fff 64bit pref]
[ 1.682775] pci 0000:01:00.0: VF BAR 3 [mem 0x4000220000-0x400023ffff 64bit pref]: contains BAR 3 for 8 VFs
[ 1.766197] pcifront pci-0: claiming resource 0000:01:00.0/0
[ 1.766201] pcifront pci-0: claiming resource 0000:01:00.0/2
[ 1.766202] pcifront pci-0: claiming resource 0000:01:00.0/3
[ 1.766202] pcifront pci-0: claiming resource 0000:01:00.0/6
[ 1.766203] pcifront pci-0: claiming resource 0000:01:00.0/7
[ 1.766204] pcifront pci-0: claiming resource 0000:01:00.0/10
[ 1.881891] igb 0000:01:00.0: Xen PCI mapped GSI16 to IRQ24
[ 1.902032] igb 0000:01:00.0: enable msix get err ffffff8e
[ 1.902042] igb 0000:01:00.0: Xen PCI frontend error: -114!
[ 2.134411] igb 0000:01:00.0: added PHC on eth0
[ 2.135751] igb 0000:01:00.0: Intel(R) Gigabit Ethernet Network Connection
[ 2.135753] igb 0000:01:00.0: eth0: (PCIe:5.0Gb/s:Width x4) 7c:c2:55:67:7e:98
[ 2.136495] igb 0000:01:00.0: eth0: PBA No: 010000-000
[ 2.136496] igb 0000:01:00.0: Using MSI interrupts. 1 rx queue(s), 1 tx queue(s)
[ 2.425477] igb 0000:01:00.0 Pskel0: renamed from eth0
[ 5.775313] igb 0000:01:00.0 Pskel0: igb: Pskel0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX

Full kernel boot logs are attached to the Ubuntu bug report but I can duplicate them here if
requested: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2138432

This has happened with xen versions 4.19.3 and 4.20.2, the dom0 kernel version
doesn't seem to matter and the pci devices work as expected when this commit
is included.

Thanks,
James