The activation for Scan at Field (SAF) includes a parameter to make
microcode wait for both threads to join. It's preferable to perform an
entry rendezvous before the activation to ensure that they start the
`wrmsr` close enough to each other. In some cases it has been observed
that one of the threads might be just a bit late to arrive. An entry
rendezvous reduces the likelihood of these cases occurring.

Add an entry rendezvous to ensure the activation on both threads happen
close enough to each other.
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20240125082254.424859-6-ashok.raj@intel.comReviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>