Re: [PATCH] libxl: enable Xen's e820_host setting
On 4/9/20 7:14 AM, Daniel P. Berrangé wrote:
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 ?
Oh, right. Thanks for the reminder! I'll have to check but I suspect it will.
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.
Nod. Do you have any suggestions on how to model this setting in libvirt? I
proposed adding a hypervisor feature for Xen in this thread
https://www.redhat.com/archives/libvir-list/2020-April/msg00376.html
rational being that for PV guests the hypervisor serves as the BIOS and provides
the facility to report the memory map to the OS. I couldn't really think of a
good fit for it within the <os> element and its children.
Regards,
Jim