This is a suggestion on a small performance improvement.
Ulrich Drepper wrote:
> int $0x80 -> call *%gs:0x18
The calling convention has been (slightly) changed - i.e. 6 argument
calls don't work, so why not go a bit further: allow the vsyscall entry
point to clobber more GPRs?
I see 3 pushes and pops in the vsyscall page (if I've looked at the
correct patch from Linus), to preserve %ecx, %edx and %ebp:
vsyscall:
pushl %ebp
pushl %ecx
pushl %edx
0:
movl %esp,%ebp
sysenter
jmp 0b
popl %edx
popl %ecx
popl %ebp
ret
The benefit is that this allows Glibc to do a wholesale replacement of
"int $0x80" -> "single call instruction". Otherwise, those pushes are
completely unnecessary. It could be this short instead:
vsyscall:
movl %esp,%ebp
sysenter
jmp vsyscall
ret
It is nice to be able to use the _exact_ same convention in glibc, for
getting a patch out of the door quickly. But it is just as easy to do
that putting the pushes and pops into the library itself:
Instead of
int $0x80 -> call *%gs:0x18
Write
int $0x80 -> pushl %ebp
pushl %ecx
pushl %edx
call *%gs:0x18
popl %edx
popl %ecx
popl %ebp
It has exactly the same cost as the current patches, but provides
userspace with more optimisation flexibility, using an asm clobber
list instead of explicit instructions for inline syscalls, etc.
Cheers,
-- Jamie
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