[PATCH] libxl: enable Xen's e820_host setting

[Jim Fehlig]

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