On Sunday 13 February 2011, H. Peter Anvin wrote: > We prototyped using the int $0x80 system call entry point. However, > there are two disadvantages: > > a. the int $0x80 instruction is much slower than syscall. An actual > i386 process can use the syscall instruction which is disambiguated > by the CPU based on mode, but an x32 process is in the same CPU mode > as a normal 64-bit process.
Well, you could simply change entry.S to allow syscall with high numbers to have the same effect as int $0x80, but not introduce another table to solve this. > b. 64-bit arguments have to be split between two registers for the > i386 entry points, requiring user-space stubs. 64 bit arguments are very rare, and most of those syscalls are not performance critical, so this could be dealt with on a case-by-case basis, possibly by introducing a new syscall number for the variant passing a 64 bit register. > All in all, the cost of an extra system call table is quite modest. The memory size overhead may be small, but auditing another table for every change could become a noticable burden (your though, not mine). > The cost of an entire different ABI layer (supporting a new memory layout) > would be enormous, a.k.a. "not worth it", which is why the memory layout > of kernel objects needs to be compatible with i386. Right, this makes sense, you certainly can't redefine all the data structures. What would probably be a good idea is to compare the set of syscalls in X32 and asm-generic, and to either eliminate or document the differences. You can probably even take the asm-generic syscall numbers, even if you keep the i386 data structures. Arnd