Linus Torvalds writes: (Summary) That sucks, because you have two choices:
- either pointlessly generate extra code for the 200+ system calls that are *not* used by the kernel
that are *not* used by the kernel
- or let gcc just merge the two, and make code generation suck where the real system call just loads the registers and jumps to the common code.
code.
That second option really does suck, because if you let the compiler just generate the _single_ system call, it will do the "load actual value from ptregs" much more nicely, and only when it needs it, and schedules it all into the system call code.
schedules it all into the system call code.
So just making the rule be: "you mustn't call the SYSCALL_DEFINEx() functions from anything but the system call code" really makes everything better.
everything better.
Then you only need to fix up the *handful* of so system calls that actually have in-kernel callers.
actually have in-kernel callers.
Many of them end up being things that could
- either pointlessly generate extra code for the 200+ system calls that are *not* used by the kernel
that are *not* used by the kernel
- or let gcc just merge the two, and make code generation suck where the real system call just loads the registers and jumps to the common code.
code.
That second option really does suck, because if you let the compiler just generate the _single_ system call, it will do the "load actual value from ptregs" much more nicely, and only when it needs it, and schedules it all into the system call code.
schedules it all into the system call code.
So just making the rule be: "you mustn't call the SYSCALL_DEFINEx() functions from anything but the system call code" really makes everything better.
everything better.
Then you only need to fix up the *handful* of so system calls that actually have in-kernel callers.
actually have in-kernel callers.
Many of them end up being things that could