Aragorn wrote:

> Xen, the Xen kernel sources and the Xen tools are masked as well in
> Portage, but I could install them nevertheless by adding them to
> */etc/portage/package.keywords* without the - in my case - "~amd64" and
> then simply emerging them. This is with the latest version of Portage.

Yes, this exploits the "missing keyword equals unstable" feature, as per man
portage:

package.keywords
Per-package KEYWORDS. Useful for mixing unstable
packages in with a normally stable system or vice
versa. This will allow ACCEPT_KEYWORDS to be augmented
for a single package.

Format:
- comment lines begin with # (no inline comments)
- one DEPEND atom per line followed by additional
KEYWORDS
- lines without any KEYWORDS imply unstable host arch

However, this was not true in the past and might change in the future as
well. Thus it's always better to explicitly specify the architecture
(~amd64 in your case).