CONFIG_XEN_VIRTIO{_FORCE_GRANT} interferes with nested virt

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Hi,

When booting Xen with Linux dom0 nested under KVM,
CONFIG_XEN_VIRTIO_FORCE_GRANT=y makes it unable to use virtio devices
provided by L0 hypervisor (KVM with qemu). With PV dom0, grants are
required for virtio even if just CONFIG_XEN_VIRTIO is enabled.

This is probably uncommon corner case, but one that has bitten me in my
CI setup... I think Xen should set smarter
virtio_require_restricted_mem_acc(), that enforces it only for devices
really provided by another Xen VM (not by the "outer host"), but I'm not
sure how that could be done. Any ideas? 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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