Re: A few XML modeling questions

Hi All,

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.".