Setting constant-time mode CPU flag

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Intel chips (Ice Lake and later) and ARM64, a bit needs to be set in
a CPU register to enforce constant-time execution.  Linux plans to set
this bit by default; Xen should do the same.  See
https://lore.kernel.org/lkml/YwgCrqutxmX0W72r@xxxxxxxxx/T/ for details.
I recommend setting the bit unconditionally and ignoring guest attempts
to change it.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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