IRET in particular is a very slow instruction.
As far as I know, though, the SYSENTER patch didn't deal with several of
the corner cases introduced by the generally weird SYSENTER instruction
(such as the fact that V86 tasks can execute it despite the fact there
is in general no way to resume execution of the V86 task afterwards.)
In practice this means that vsyscalls is pretty much the only sensible
way to do this. Also note that INT 80h will need to be supported
indefinitely.
Personally, I wonder if it's worth the trouble, when x86-64 takes care
of the issue anyway :)
-hpa
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