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

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Message-ID: <aWkqA3u6E1fwB9vj@Mac.lan>
Date: Thu, 15 Jan 2026 18:55:15 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: James Dingwall <james@...gwall.me.uk>
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 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?
> > 
> 
> Here's the .config, there are some deviations from the default Ubuntu build
> but it isn't too far off.
> 
> Thanks,
> James
> 
> #
> # Automatically generated file; DO NOT EDIT.
> # Linux/x86 6.8.12 Kernel Configuration
[...]
> #
> # Xen driver support
> #
> CONFIG_XEN_BALLOON=y
> CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y
> CONFIG_XEN_MEMORY_HOTPLUG_LIMIT=512
> CONFIG_XEN_SCRUB_PAGES_DEFAULT=y
> CONFIG_XEN_DEV_EVTCHN=m
> CONFIG_XEN_BACKEND=y
> CONFIG_XENFS=m
> CONFIG_XEN_COMPAT_XENFS=y
> CONFIG_XEN_SYS_HYPERVISOR=y
> CONFIG_XEN_XENBUS_FRONTEND=y
> CONFIG_XEN_GNTDEV=m
> CONFIG_XEN_GNTDEV_DMABUF=y
> CONFIG_XEN_GRANT_DEV_ALLOC=m
> CONFIG_XEN_GRANT_DMA_ALLOC=y
> CONFIG_SWIOTLB_XEN=y
> CONFIG_XEN_PCI_STUB=y
> CONFIG_XEN_PCIDEV_BACKEND=m
> CONFIG_XEN_PVCALLS_FRONTEND=m
> # CONFIG_XEN_PVCALLS_BACKEND is not set
> CONFIG_XEN_SCSI_BACKEND=m
> CONFIG_XEN_PRIVCMD=m
> CONFIG_XEN_PRIVCMD_EVENTFD=y
> CONFIG_XEN_ACPI_PROCESSOR=y
> CONFIG_XEN_MCE_LOG=y
> CONFIG_XEN_HAVE_PVMMU=y
> CONFIG_XEN_EFI=y
> CONFIG_XEN_AUTO_XLATE=y
> CONFIG_XEN_ACPI=y
> CONFIG_XEN_SYMS=y
> CONFIG_XEN_HAVE_VPMU=y
> CONFIG_XEN_FRONT_PGDIR_SHBUF=m
> CONFIG_XEN_UNPOPULATED_ALLOC=y

Can you confirm that the config used to build the non-working kernel
also has CONFIG_XEN_UNPOPULATED_ALLOC=y?

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

Thanks, Roger.