linux-kernel - Re: xen pci passthrough stops working after xen/x86: fix initial memory balloon target

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Message-ID: <aWoEc7Lxh0zG62S1@dingwall.me.uk>
Date: Fri, 16 Jan 2026 09:27:15 +0000
From: James Dingwall <james@...gwall.me.uk>
To: Roger Pau Monné <roger.pau@...rix.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: xen pci passthrough stops working after xen/x86: fix initial
 memory balloon target

On Thu, Jan 15, 2026 at 06:55:15PM +0100, Roger Pau Monné wrote:
> On Thu, Jan 15, 2026 at 02:50:12PM +0000, James Dingwall wrote:
> > On Thu, Jan 15, 2026 at 01:03:49PM +0100, Roger Pau Monné wrote:
> > > On Thu, Jan 15, 2026 at 11:23:37AM +0000, James Dingwall wrote:
> > > > Hi,
> > > > 
> > > > We have encountered a regression with pci passthrough since the
> > > > Ubuntu 6.8.0-91.92 which included this commit:
> > > 
> > > Hello,
> > > 
> > > Thanks for the report.  Could you also send me your kernel Kconfig, to
> > > see which combination of options are you using?
> > > 
> > 
> Can you confirm that the config used to build the non-working kernel
> also has CONFIG_XEN_UNPOPULATED_ALLOC=y?

The config is the same for both builds and CONFIG_XEN_UNPOPULATED_ALLOC=y
is always set. 

> Can you also provide the output of `cat /proc/iomem` for both the
> working and non-working kernels?

non-working Ubuntu-6.8.0-100.100:

00000000-00000fff : Reserved
00001000-0009ffff : System RAM
000a0000-000fffff : Reserved
  000f0000-000fffff : System ROM
00100000-2007ffff : System RAM
  01000000-025fffff : Kernel code
  02600000-033bcfff : Kernel rodata
  03400000-0385613f : Kernel data
  03d54000-041fffff : Kernel bss
20081000-73b57fff : Unusable memory
76c58000-76d76fff : ACPI Tables
76d77000-76ea0fff : ACPI Non-volatile Storage
77fff000-77ffffff : Unusable memory
80000000-87ffffff : System RAM
88000000-8fffffff : Xen scratch
100000000-103f7ffff : System RAM
4000200000-400021ffff : 0000:01:00.0
4000220000-400023ffff : 0000:01:00.0
4000240000-400025ffff : 0000:01:00.1
4000260000-400027ffff : 0000:01:00.1


working Ubuntu-6.8.0-100.100:

00000000-00000fff : Reserved
00001000-0009ffff : System RAM
000a0000-000fffff : Reserved
  000f0000-000fffff : System ROM
00100000-2007ffff : System RAM
  01000000-025fffff : Kernel code
  02600000-033bcfff : Kernel rodata
  03400000-0385613f : Kernel data
  03d54000-041fffff : Kernel bss
20081000-73b57fff : Unusable memory
76c58000-76d76fff : ACPI Tables
76d77000-76ea0fff : ACPI Non-volatile Storage
77fff000-77ffffff : Unusable memory
81100000-811fffff : 0000:01:00.1
  81100000-811fffff : igb
81200000-812fffff : 0000:01:00.0
  81200000-812fffff : igb
81300000-8137ffff : 0000:01:00.1
81380000-813fffff : 0000:01:00.0
81400000-81403fff : 0000:01:00.1
  81400000-81403fff : igb
81404000-81407fff : 0000:01:00.0
  81404000-81407fff : igb
81500000-815fffff : 0000:03:00.0
81600000-816fffff : 0000:03:00.0
  81600000-816fffff : igc
81700000-81703fff : 0000:03:00.0
  81700000-81703fff : igc
88000000-8fffffff : Xen scratch
100000000-103f7ffff : System RAM
4000200000-400021ffff : 0000:01:00.0
4000220000-400023ffff : 0000:01:00.0
4000240000-400025ffff : 0000:01:00.1
4000260000-400027ffff : 0000:01:00.1


Just for completeness the working build also reverts "xen/x86: fix initial
memory balloon target" because of a conflict in drivers/xen/balloon.c.

Thanks,
James