Re: [PATCH 6/7, RFC] x86_64: basic changes for supporting compatibility mode guest

["Jan Beulich" <jbeulich@novell.com>]

There was one more issue I forgot to mention: We must disable execution of
'syscall' in some way for compatibility domains. The question is whether to
add respective detection code early into the syscall path (which would affect
performance-wise all 64-bit domains) or whether to {dis,en}able the use of
the instruction in the context switch code by updating EFER as needed
(which would have a performance effect only when switching between
a native and a compatibility domain, but the performance effect would likely
be significantly greater). I'd prefer the second option, not the least because
its implementation is significantly easier.

Jan