A few XML modeling questions

Hi All, I've been struggling a bit deciding how to model Xen's max_event_channels and e820_host xl.cfg(5) settings. For max_event_channels the man page says: Limit the guest to using at most N event channels (PV interrupts). Guests use hypervisor resources for each event channel they use. The default of 1023 should be sufficient for typical guests. The maximum value depends on what the guest supports. Guests supporting the FIFO-based event channel ABI support up to 131,071 event channels. Other guests are limited to 4095 (64-bit x86 and ARM) or 1023 (32-bit x86). So event channels are PV interrupts that are used by PV devices and timers, and as inter-processor interrupts. My initial thought was to add a maxEventChannels attribute to the xenbus controller, similar maxGrantFrames. The latter was added to allow specifying the max shared pages a guest could use, which in conjunction with event channels provides data transfer mechanism between front and backends for PV drivers. My only hesitation is that event channels are also used for inter-processor interrupts, which is a bit outside the notion of a virtual controller for Xen PV devices. However, the first sentence in the xenbus wiki [1] states: In practice, the bus is used for configuration negotiation, leaving most data transfer to be done via an interdomain channel composed of a shared page and an event channel. So perhaps it is tolerable to consider max_event_channels an attribute of xenbus that is specified at domain creation, to then be used by the guest however it pleases up to the max value? e820_host is a bit trickier. For this setting, which is PV-specific, the man page says: Selects whether to expose the host e820 (memory map) to the guest via the virtual e820. When this option is false (0) the guest pseudo-physical address space consists of a single contiguous RAM region. When this option is specified the virtual e820 instead reflects the host e820 and contains the same PCI holes. The total amount of RAM represented by the memory map is always the same, this option configures only how it is laid out. Exposing the host e820 to the guest gives the guest kernel the opportunity to set aside the required part of its pseudo-physical address space in order to provide address space to map passedthrough PCI devices. It is guest Operating System dependent whether this option is required, specifically it is required when using a mainline Linux ("pvops") kernel. This option defaults to true (1) if any PCI passthrough devices are configured and false (0) otherwise. If you do not configure any passthrough devices at domain creation time but expect to hotplug devices later then you should set this option. Conversely if your particular guest kernel does not require this behavior then it is safe to allow this to be enabled but you may wish to disable it anyway. I'm tempted to unconditionally enable this setting. It is required for pvops kernels and apparently harmless for other PV kernels. I asked one of the Xen devs about any downsides to always enabling e820_host, to which he replied "Scattered memory blocks inside the guest, possibly leading to slightly higher overhead. But nothing really severe afaics.". If there is a strong desire to model this setting it might best be placed under hypervisor features, e.g. <features> <xen> <hoste820 state='on'/> </xen> </features> Opinions and/or comments are much appreciated. Regards, Jim [1] https://wiki.xen.org/wiki/XenBus