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SubjectRE: [perfmon2] comments on Performance Counters for Linux (PCL)
>> Happen to have more details? If they really run in user context their
>> priv level would be the same right, otherwise they run in something
>> weird, not quite user not quite kernel. Now that might be the case, but
>> I'm utterly ignorant on itanic.
>
> See Documentation/ia64/fsys.txt

Summary is that the fast system calls run in ring 0 ... but since they
didn't save all the user state (in order to make them fast!) there are
restrictions on what things they can do. In particular they must not
trap.

-Tony


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