It loses because it throws away %eip and %esp, requiring those values
to be saved in user space, and giving you no indication where you came
from. Oh yes, and it loses because it works from V86 mode, with no
way to tell you were coming from V86 mode and hence shouldn't execute
arbitrary system calls. The only way around that is to zero the
SYSENTER MSR *in the scheduler*, slowing down a common path because
you can't put a needed test in an uncommon path.
(Actually, there is a better alternative: the vm86() system call
should set a flag in the process structure, and the signal handlers
should clear it; if you get a SYSENTER with the flag set you know you
were in V86 mode. Note that since %cs, %ss, %eip and %esp are
irrecoverably lost, all you can do is shut the V86 process down, or
emulate an 8086 reset.)
-hpa
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I am Bahá'í -- ask me about it or see http://www.bahai.org/
"To love another person is to see the face of God." -- Les Misérables
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