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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 > > > > For some reason (which I still haven't figure out), the fictitious PFN memory layout > > created by Linux ends up placing a RAM region over the BAR MMIO space > > used by igc, the difference: > > > > 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 > > > > VS > > > > 80000000-87ffffff : System RAM > > 88000000-8fffffff : Xen scratch > > 100000000-103f7ffff : System RAM > > > > In the non-working case there's a chunk of RAM in the space that > > covers the device MMIO BARs. I fear my balloon accounting "fix" has > > instead introduced a miss accounting in the balloon driver that causes > > Linux to attempt to balloon up memory and it ends up instantiating a > > hotplug memory region over the device MMIO BARs. > > > > I'm still confused as to how the change in balloon_add_regions() has > > an effect when CONFIG_XEN_UNPOPULATED_ALLOC=y, as it should become a > > no-op in that case, but I will debug this myself. > > > > > > > > Just for completeness the working build also reverts "xen/x86: fix initial > > > memory balloon target" because of a conflict in drivers/xen/balloon.c. > > > > Hm, but that's the same commit that you mentioned in the first email, > > there you said reverting: > > > > commit 74287971dbb3fe322bb316afd9e7fb5807e23bee > > Author: Roger Pau Monne > > Date: Wed May 14 10:04:26 2025 +0200 > > > > xen/x86: fix initial memory balloon target > > > > Fixes the issue. Is there an additional commit that also needs > > reverting to fix the issue? That would make more sense, as IMO that > > commit should be a no-op given your Kconfig. > > Argh! This is my mistake reading the two reverts in the wrong order. The > bisect landed on "x86/xen: fix balloon target initialization for PVH dom0" > but "xen/x86: fix initial memory balloon target" had to be reverted first. > I'm sorry if that left you scratching your head. > > > I don't think I will be able to get into this until Monday, sorry. In > > the meantime, does disabling the balloon driver mitigate the issue? > > I can try this if it could still be relevant? I'm positive it will workaround the issue, by simply not enabling the balloon driver. I don't think it will specially help me debug it, but it should get you going without needing a custom build kernel (if you don't care about ballooning). Hopefully I will be able to get back to you on Monday with a fix for the issue, sorry for the inconvenience this might have caused, and thanks for bisecting it. Regards, Roger.