[PATCH] libxl: enable Xen's e820_host setting

Daniel P. Berrangé berrange at redhat.com
Thu Apr 9 13:14:47 UTC 2020

On Wed, Apr 08, 2020 at 02:29:16PM -0600, Jim Fehlig wrote:
> Hotplugging PCI devices to Xen PV guests is only possible if the
> libxl_domain_build_info struct has the e820_host field enabled when the
> guest is created. By default it is disabled but libxl will automatically
> enable e820_host if the config contains one or more PCI devices, in which
> case hotplugging additional PCI devices later works.
> According to xl.cfg(5) man page it is safe to unconditionally enable the
> PV-only e820_host setting. Furthermore xen.git commits 414979ba85 and
> f92337d949, which introduce the setting with a default of disabled, claim
> the setting can be enabled or even removed "once the auto-ballooning of
> guests with PCI devices works". Those commits are from May 2011 so I
> think it is safe to say the issues have been resolved in the meantime.
> Regardless, we should avoid exposing a Xen setting in libvirt that could
> be removed later.

Does this have any implications for live migration compatibility if you
silently enable this for all guests ?

In QEMU/KVM if you did this, it would be considered an ABI change and
could break live migration of a guest launched on old libvirt, to a
host running new libvirt.

> Signed-off-by: Jim Fehlig <jfehlig at suse.com>
> ---
> See related thread on the ML
> https://www.redhat.com/archives/libvir-list/2020-April/msg00376.html
>  src/libxl/libxl_conf.c                        | 5 +++++
>  tests/libxlxml2domconfigdata/basic-pv.json    | 3 ++-
>  tests/libxlxml2domconfigdata/multiple-ip.json | 3 ++-
>  3 files changed, 9 insertions(+), 2 deletions(-)

*if* the live migration compatibility is not a problem, then

Reviewed-by: Daniel P. Berrangé <berrange at redhat.com>

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