A few XML modeling questions

[Jim Fehlig]

On 4/7/20 12:02 PM, Jim Fehlig wrote:
> 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.".

I dug a bit deeper to find the xen.git commits that introduced e820_host: 
414979ba85 and f92337d949. The latter, from May 2011, claims the setting can be 
removed "once the auto-ballooning of guests with PCI devices works"

https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=f92337d9494efbab0918eb0e055d129fce5c3227

I think this is further evidence that we should unconditionally enable e820_host 
in libvirt and avoid exposing a Xen setting that might be removed in the future.

Regards,
Jim