[PATCH] libxl: enable Xen's e820_host setting

Jim Fehlig jfehlig at suse.com
Tue Apr 14 21:56:47 UTC 2020

On 4/13/20 1:17 PM, Marek Marczykowski-Górecki wrote:
> FWIW, in Qubes we have a patches adding e820_host setting here:
> https://github.com/QubesOS/qubes-core-libvirt/
> (patches 8-11)
> Not submitted before, exactly to avoid adding temporary options. But
> since 8+ years later it is still there, I think it's safe to assume it
> will be there for some more. Or at least it's worth to unbreak some
> configurations in the meantime.

BTW, the other piece of the puzzle for PCI passthrough starting with xen 4.13 is 
the xl.cfg 'passthrough' option. See xen commit babde47a3fe. Without that 
enabled, hotplugging a PCI device to an HVM or PV domain is not possible, with 
or without e820_host. AFAICT there are no patches in your queue for the 
'passthrough' option. Perhaps you haven't rebased to xen 4.13. Do you have any 
thoughts on modeling that option?

My thoughts are adding another xen hypervisor feature (similar to e820_host) or 
map it to the new IOMMU device in libvirt [1]. I suppose it depends on the need 
to support the {sync,share}_pt values vs a simple enable/disable. If enabling 
and disabling passthrough is enough, I'd lean towards another xen hypervisor 
feature. Otherwise we'll need to explore the IOMMU device?


[1] https://libvirt.org/formatdomain.html#elementsIommu

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