In long mode the %cs is largely a relic. However there are a few cases
like lret where it matters that we have a valid value. Without this
patch it is possible to enter the kernel in startup_64 without setting
%cs to a valid value. With this patch we don't care what %cs value
we enter the kernel with, so long as the cs shadow register indicates
it is a privileged code segment.
Thanks to Magnus Damm for finding this problem and posting the
first workable patch. I have moved the jump to set %cs down a
few instructions so we don't need to take an extra jump. Which
keeps the code simpler.
Signed-of-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>